Archive of Our Own

♠️

Asagiri Gen is a fantastic lay.

He knows how he looks with his uncannily pretty face and his foxy smile. He knows how to coax without seeming to coax, to persuade with the tilt of his chin. He looks aimless. He gets exactly what he wants.

He knows how to make it good: to give it not too hard but hard enough; to moan and gasp and touch his own chest when he takes it. To whisper desire like he means it. To shudder like he’s overcome.

Everybody has a good time. Everyone leaves happy.

Trust Senku to be unlike anyone else.

Senku is eighteen, give or take thirty-seven hundred years, when they meet in the stone world; he’s eighteen when he marries, and promptly divorces, the most sincerely beautiful girl in the village. He doesn’t want for opportunities, and shows no romantic or sexual interest in anyone. Ordinarily Gen would give it up as a bad job and find another partner.

But despite being almost certainly a virgin and possibly asexual, Senku has the biggest dick energy Gen’s ever seen. He walks around with the unshakeable confidence of a man who has never once measured himself and found himself lacking. There must be a reason why he opted for a science dress over science pants.

Gen likes a big dick. Gen loves knowing things that no one knows.

No one knows how Senku fucks.

And if anyone can talk himself into a ride, it’s Asagiri Gen.

None of the usual tricks work. Innuendoes soar over Senku's brilliant head. Familiar little touches to his back and wiry arms and hands are permitted, but ignored.

The observatory makes Senku’s eyes shine, but as far as getting into his pants, it goes over like a lead balloon.

Gen isn’t surprised, and he isn’t discouraged. But he does start to wonder what, short of stripping naked and mounting the lab table, can be done to show Senku that Gen’s available and very interested.

Really, he should have known better than to assume Senku hadn’t noticed.

The weather is warm but it’s getting dark earlier, so the village eats dinner clustered together around a large crackling fire. Tonight it’s rabbit and beans, practically a Michelin-star meal in the stone world. Senku's set aside the barley roll that he always takes but never eats.

Gen wants it.

He goes about it in the usual way. He puts on a minor spectacle enjoying his own barley roll; he magnanimously offers Senku the last half of his beans. Nothing. Senku ignores him.

He’s weighing the prudence of just palming the roll as he passes Senku on his way from the fireside when Senku looks at him drily and says, “Hoy, mentalist. You're ten billion percent more likely to get what you want if you ask for it.”

He is and isn't talking about the barley roll.

Gen knows his mouth is hanging open unattractively and snaps it shut. His first instinct is to bluff his way out. But he’d be a fool to try to talk his way through a vault door when Senku’s just handed him the key.

So instead he thinks for a moment, and then says lowly: “Do you have time for me tonight, Senku-chan?”

He can't help the flip his stomach takes when Senku simply says, “Yes.”

He hands Gen the barley roll and grins.

♠️♠️

As Gen deduced, Senku has never had sex.

Gen doesn't hold it against him. Senku had never manufactured an antibiotic, built a cell phone, or waged a war. He mastered all three in less than a year. Sex should be comparatively easy.

In a past life, Gen lived from stage to stage. He has never felt so scrutinized. They sit, knees together, on the plank floors of the observatory and Senku touches him one piece at a time: his palms, his hair, his mouth. With each he watches Gen blink or twitch or breathe until he seems satisfied, and then it's on to the next.

Senku's approach to sex is much the same as his approach to anything else. It’s deliberate and methodical, and it doesn't resemble anything Gen would identify as sex. The whole process is far too unhurried, prone to stops and starts, nothing building.

But he recognizes the procedure: start with the raw essentials and refine, refine, refine. So he allows Senku his leisurely probing. Now his ears; his throat; the dip of his clavicle. Under Gen’s trousers his cock lies soft against his thigh, but it's dormant rather than unmoved.

It's a quiet night. The village sleeps early in the lull between one project winding down and another that hasn't yet gained momentum. A warm breeze stirs the spruce trees outside the open panels of the observatory’s dome roof, raising goosebumps on the bare skin of Gen's arms; earlier, he’d taken off his yukata with a flourish that made Senku grin.

They have the observatory to themselves. They have electric light, and time.

Senku's curious touches take a turn. He begins to revisit parts previously inspected and disposed of. A barely-there fingertip pets Gen's right palm. A hand smooths the length of his spine, drawing a little shiver that Gen’s quick to suppress. Senku looks him in the eye for the first time since they began and grins, triumphant.

Gen takes his wrist and kisses him.

Senku’s clever mouth is firm and unyielding under his own. Gen keeps at him, pulling gently at his lips until they soften and give. And then they're kissing.

Senku’s a quick study. Whatever Gen does, he takes and gives it back, experimental and bold. When Gen’s satisfied that he has the basics down, he gives him a little more. His tongue steals into Senku’s mouth. A moment later, Senku’s giving it back to him and he’s giving it back better.

So Gen pulls out all the stops. He climbs into Senku’s lap, mouth open and eyelids low, to kiss him, then nudges him down. Senku watches him with rapt attention, propping himself up on his elbows for a better view even as he stretches himself back on the plank floor. Gen gives himself a moment to appreciate the long, bony lines of him, the breadth of his shoulders in his funny science dress.

While the audience is new, the venue is familiar. Senku’s hard now, so Gen gives his dick an extra little grind to say welcome to the show.

And what a show it is. He runs a messy hand through his hair and then, eyelashes lowered, lets it fall into his face as he unlaces his shirt with languid fingers. Senku’s eyes fall away from his face to his bare chest. It’s a relief. He keeps Senku’s attention there, bringing his fingers up to his collarbone, drawing them down to his nipple.

Senku’s mouth parts and he asks, a little hoarsely, “Does that feel good?”

Gen answers him by pinching Senku’s own nipple hard through his clothes, making Senku buck and shout. He’s dumped laughing to the floor between Senku’s knees but manages to turn it into a sexy tumble.

He knows very suddenly what he wants Senku to see. He slides his hands under Senku’s thighs. “Pick up your knees, Senku-chan.”

Senku casually scratches his neck. “Why?”

Gen’s sure he'll be hearing that a lot tonight. It’s no less annoying for its predictability. Instead of sighing, he winks. “I'll show you.” Senku studies him for a long moment until curiosity wins out.

Slim and wiry as his thighs are, they’re sturdy and surprisingly heavy. Senku must know what comes next but he’s utterly unfazed, content to let Gen take the whole weight of his legs and sling them over his shoulders.

It pisses Gen off that he’s so relaxed. He wants to shake him up. He wants Senku’s surprise, hard-won from the man who knows how everything works. It makes him want it more.

“No one's ever blown you, have they, Senku-chan?” He gives Senku his sleepiest bedroom eyes. His mouth parts, a promise implied.

“No,” Senku says. His thighs finally tense a little.

It’ll do. “Good,” Gen says with a cruel smile, and means it. He flips up Senku’s hem, ducks his head and swallows him down.

It’s a stretch. Gen gives himself a moment to feel the deep satisfaction of a man who has accurately ferreted out a massive cock. Then he gets to work.

At first it’s easy to keep Senku where Gen wants him. He gives a master class in outstanding sloppy head and revels in Senku’s heavy panting breaths. Senku’s so new to this, so easily moved by sensation. It makes him malleable. Gen can direct Senku’s attention to his cock or his balls, his mouth or his hand; he can pull out moans like silks from a hat; push him to the edge and coax him back. After a few minutes of being played this way, Senku lifts a cautious hand. He smooths a thumb along Gen’s cheekbone, pushing his hair away from his face.

This isn’t new, exactly. Men like to watch Gen when he's sucking them off. They like to see their cocks in his mouth through his soft pale cheek. Sometimes they touch his face to feel the head inside him. When they do, Gen closes his eyes, makes it look prettier than it is. He’s mesmerizing. He’s powerful. They can’t look away.

Senku’s watchfulness is not that kind. Gen’s back under the microscope again.

So he does what he does best. He gives him a hell of a show.

Lashes lowered, he takes long, greedy sucks and moans hungry in the back of his throat. He pulls off for a second so Senku can see his sloppy mouth. Lets the electric light shine off the string of spit between his lip and Senku’s dick. Sucks a drip of it off his own thumb. It’s cold and gross. It looks fucking hot.

Senku is spellbound. “Mentalist,” he murmurs. “Gen, I—”

Gen doesn’t want to hear the end of that sentence. He cuts him off by taking him back down his throat. That does it. Senku stops talking and gives himself up.

He comes with a shocked hiss. It doesn't taste too bad; something about the stone world diet, Gen supposes. He releases Senku to pant against the floorboards and wipes his mouth. Pretty again.

Senku reaches for him. When Gen doesn't come right away he grabs him and pulls him down. Gen, because he is a fool, lets him.

In the aftermath of his orgasm Senku kisses lazily but with no less precision than he had during their lesson. Gen, ten million times a fool, forgets himself in kissing him back. He winds around Senku, relaxed, easy. He thinks he’s safe while Senku’s still come-drunk and careless.

He’s proved hilariously wrong when Senku shoves down Gen’s trousers and gets a hand around him. It takes all he’s got in him not to go stiff with surprise.

It’s more peculiar than pleasurable. Senku turns his dick over to inspect it from all sides, prods at Gen’s balls and strokes him at infrequent intervals. A fingertip poking in his sensitive urethra makes him yelp. He bats at Senku’s head, but Senku shoves him back and won’t be shifted.

And that’s a problem, because Senku suddenly puts it all together. The little experimental touches become long strokes fast and rough over the head. Gen bites his lip to keep from groaning because it’s so fucking good. It’s worse because it’s so unexpected, a moment of sexual eureka! that he knows better than to assume will be Senku’s last. And if this is how Senku starts, then—

Fuck—

It’s almost okay while Senku’s still staring at his hand on Gen’s dick, but when he looks up to study his face, it’s too much. He closes his eyes and kisses Senku, and pretends it’s to keep himself quiet against waking the village.

All at once Senku stops kissing and touching him.

He opens his eyes. Senku’s studying his own palm. To Gen’s dismay, he licks it wet.

The long rough strokes go too smooth, feel too good. Senku tightens his fist and looks up.

“How does that feel?” His eyes are hot on Gen’s.

It feels like something he’s had a thousand times over. It feels completely new.

It’s horrifying. He closes his eyes against Senku’s burning gaze and comes helplessly all over his own belly.

He doesn’t bother to dress before he bolts.

♠️♠️♠️

Gen concedes that he’s fucked up spectacularly.

He considers not returning to Senku at all. But the gravity between them is too strong, and Gen is a weak man. When Senku gives him an intent look over a diagram for the hot air balloon, he’s pulled helplessly in.

Everyone but Senku leaves the laboratory after the meeting ends.

Only Gen returns.

They end up on the floor of the lab, Gen flat on his back. He’s not totally sure how he got there. He tries to be annoyed about it, but he’s distracted. Senku’s touching him again, exploring places he hadn’t reached the last time they’d been— together, starting with Gen’s filthy feet and working his way up.

Senku's brows draw together when he reaches Gen's inner thighs.

“Why are you—” He rubs his fingers together. “Males don't produce natural lubrication. Why are you greasy?”

“That's unflattering,” Gen sighs. “But yes, you're right. When men fuck—” he lets the k pop on fuck “—they need something to ease the way. I did it before I came.” Insurance. Just like loading the gimmick that makes the whole trick work.

Unspoken, but heavily, heavily implied: fuck me, Senku-chan.

Senku's lips part on a surprised huff. He raises his fingers and sniffs, then tastes.

“Safflower oil?” Gen smiles and doesn’t answer. “Next time, wait until you're here. I want to watch you do it.”

Just like Senku to have no doubt that this wouldn't be the last time if he wanted it.

He’s right. Gen orbits Senku’s desires like a small, dark moon, and waits for the explorer to land. Thinking about it makes him feel wild and uncontrolled. He hates it.

The feeling flees when Senku touches a fingertip to his asshole. “There you are,” he mutters to himself.

“Senku-cha—” Gen stumbles as he's breached. “You don't need to, I did it for you. Fuck me, already. Just— fuck me.” He parts his mouth on a soft sigh, makes his eyes go liquid. “Please.”

“For me?” Senku muses. From anyone else, it'd be repetition of the dirty talk they liked most. From Senku, it's a proposition leading into a syllogism. Gen steels himself. “No, you just wanted to do it alone. Which begs the question.” He meets Gen’s eyes.

Gen is frozen, a beetle on a pin, a specimen on a slide. And then Senku curls his finger up, and in, and the gut-punch of feeling has Gen hot and restless and needing to squirm.

He doesn't let himself. It’s a near thing.

Senku's eyes narrow. He does it again. And again.

Two fingers. Does it again.

This time, a little cry escapes him before he can bite it back. He claps one hand over his mouth — and realizes his mistake.

Senku lights up. There’s no other way to describe it. “You son of a bitch. You’ve been bullshitting me.” He’s grinning, eyes shining with a manic light. “I knew you gave me some shit last time. You left so fast that— well, not that I thought for one millimeter that you’d stay. But you could have run into anyone. You’d never let anyone see you like that, not if there wasn’t something— else.”

Bastard. Bastard. Bastard. Gen grits his teeth against the word.

Senku presses his fingers into him again, far too cleverly. A shudder rips through Gen’s unwilling body. He wishes wildly for the stone. Better than this, with everything exposed to Senku’s goddamned sharp eyes that see way too fucking much.

All right. It's fine. It’s not the first time an audience has burned Gen. The trick’s been exposed? Then he’s going to overload the scene with sheer volume of data until Senku doesn’t know what's misdirection and what’s real.

On Senku’s next press in, he gives a convincing little shimmy. “Senku-chan— ah!” Big dramatic heaving breaths, sinuously rolling his hips into Senku’s fingers. He throws his head back, a feigned response to pleasure actually felt. “Please, please—”

Senku rolls his eyes. “I'm a virgin, not an idiot. Cut it out.”

Okay. That's not everything he's got. On a writhe, he palms the little stone he keeps in his sleeve for emergencies. Big moan. In that moment, he flicks the stone.

The electric lightbulb above the door shatters spectacularly and showers the entryway with glass.

Senku doesn't even flinch. The only indication he's noticed at all is an unimpressed little huff, barely audible over the sick squelch of his fingers between Gen's thighs.

It strikes Gen like a slap. He struggles to pull away, to— what? He doesn’t know. He’ll figure it out once he gets away.

He doesn’t even make it to his feet. Senku grabs him by both ankles and pulls him down flat to the floor. He kicks, but Senku’s got a grip on him that he can’t shake. He twists and Senku pulls him until his ass is practically in Senku’s lap. Doesn’t stop touching him. Might not stop no matter what Gen does.

Something in Gen goes lax and liquid. He wants—

He wants to let Senku, and he hates it.

Senku, the son of a bitch, sees.

“Don’t run away from me, mentalist.” His eyes are hot.

Gen forces himself to be still. “Who’s running? I—”

“Stop feeding me shit. You want me to fuck you? I’ll fuck you.” Senku twists his fingers in again. Again. “But you’re going to be honest with me. I didn’t come here for a show.” Again.

This time, the word tears out of him before he can stop it: “Bastard.”

“Do you want me to stop?”

Gen pants and glares.

Senku says it again, mocking. “Do you want me to stop?”

“No!” He spits it out like something bitter.

With Senku’s fingers animating him, he feels like one of his own tricks. But there’s no gimmick, no out. No misdirection. No smooth talk, no magic words to hide behind. It’s just Senku and Gen and what Senku wants. What Senku wants, and what Gen needs.

Senku never gives up. He’ll keep at him until Gen shows him what he wants to see. Until he shows Senku the truth.

Gen gives up.

He must. He can’t fight Senku and himself, too.

“No,” he says again. It’s true.

It’s been true since he first awakened in the stone world and saw Senku’s name written on it. Senku writes his name on everything he makes. The scratches on that tablet were notice to the stone world from the man who would be its conquerer.

He doesn’t want Senku to ever stop.

Senku grins at him. He’s beautiful like this, the shadows from the electric light giving softness to his victor’s smile. He’s beautiful all the time but Gen’s better at ignoring it when he’s not two knuckles deep in him.

He doesn't feel like a specimen on a slide any longer. Senku's looking at him like he's a marvel, and it's worse.

“Now you're finally getting me excited.”

“I'm sorry,” says Gen, an idiot. “Were you not here when I sucked you off? You loved it.”

Senku shrugs.

“You— didn't?” His own eyes are hot but not with desire. He doesn’t know why he cares. Everything's coming to the surface, everything that Gen's tried to keep up his sleeve.

Senku is merciless. “It felt fine.” He slowly pushes a third finger into him. Gen gasps and clenches his fists against the feeling, against the tears still threatening. “This is ten billion percent better.”

He rubs the back of his fist over his eyes. “Yeah? Why's that, Senku-chan?”

Senku leans in and grins. “Of course I want to see the secret depth of such a shallow man, mentalist.” He crooks his fingers in Gen to underscore his double meaning.

A wet laugh bubbles out of Gen, and something else, too— an unattractive little hiccupy moan. It sounds stupid. He curls his hand against his mouth as if it could stem the flow of further stupid sounds.

Senku, damn him, isn’t having it. “Ah, ah,” he says, taking Gen’s hand from his mouth and weaving their fingers together. “How do I know what you like if you don’t give me feedback?”

Gen wordlessly gestures one-handed at his whole body. There’s a host of things to catalogue and assess: the dilation of his pupils, his heart rate, the heaviness and frequency of his breath. The way he’s rising to meet Senku’s fingers in spite of himself.

Senku hums. “Yes, but direct evidence is superior to circumstantial. Stronger correlation. And I want to hear that you like it. Tell me.”

Fuck you, Gen thinks with the last of his pride, and pinches his mouth shut.

Senku waits for a moment, then sighs. “You’re really childish about this. I don’t understand it, but I will.”

It isn’t a threat, but it feels like one.

By the time Gen’s figured out what Senku intends to do, he’s already done it. He unlaces their fingers, puts his thumb under Gen’s chin and pushes all four fingers into his mouth. Just like that, all Gen’s stupid little sounds are freed.

Senku is going to die a virgin.

Senku, blissfully unaware or uncaring of his impending doom, grins in self-satisfaction and redoubles his efforts below. He seems to delight in asking questions Gen can’t answer. Does that feel good? Obviously. Does Gen want a fourth finger? Well, he’s getting it. He’s drooling a vile puddle into Senku’s palm and he’s so hard that his cock aches.

Senku’s eyes are everywhere on him. Gen can hardly bring himself to care.

“Will you come like this?” Gen can only moan in answer. “Ha, okay.” Senku takes his fingers out of Gen’s mouth and wipes them on his hip. Gen licks the dry roof of his mouth and makes a little moue but doesn’t push it; that hasn’t been going well for him today.

Senku asks again. “Will you?”

Gen nods. He doesn't know what he'll say if he tries to speak.

It isn't long. When he does come, it's wrenching and vicious, twisting his back and cramping his feet, and he wails full-throated.

He doesn’t need to look at Senku to know he’s satisfied.

Gen is . . . something. He stares blankly at the laboratory ceiling. Except the awkward oversensitive drag of almost his entire fucking fist, what the fuck, out of Gen, there’s nothing from Senku. Giving Gen time to put himself back together, maybe.

When his breathing has evened out, Senku speaks. “How was it?”

He doesn’t have it in him to give him anything but the truth. “It was good.”

It was really good, absurdly good, maybe the best, despite the wrestling and the pinning down and the tremendous unraveling. Probably because of it, if he’s honest with himself.

Senku pets his belly briefly with his forefinger, then wipes it on his thigh. When Gen looks down he realizes Senku’s written his own name in Gen’s jizz.

Senku writes his name on everything he makes, after all.

He laughs. He can’t help it. The whole thing is ridiculous. He’s just had one of the most powerful orgasms of his life, dick-free in the middle of an emotional breakdown on the floor of a primitive laboratory. The virgin who gave it to him is still wearing all his clothes.

Well, he was wearing them. In the time since Gen came, he’s got his belt and his science dress off. Under his clothes Senku is geometric, hard angles and slim planes. Gen doesn't linger on his shoulders or his waist, but he notices them.

Senku wraps a hand around his dick and squints at it and then at Gen's ass. Gen can see his calculations come up short.

He doesn't care. Senku will fuck him or not as he decides. Maybe he'll jerk off on Gen and Gen can keep staring at the ceiling and imagine he's anywhere but the lab. Maybe Senku will take field notes afterwards. Time from arousal to plateau to orgasm. Observations of Gen’s physiological reactions. Angle and velocity of his jizz.

Instead he wonders how Senku's face will look.

He hates himself for it, a little. But he's gotta. He's gotta know.

He kicks Senku in the thigh. “Oh no, Senku-chan, I'm scared! I've never seen such a monster hog! How will my innocent body take it?” He rolls his eyes and his hips, a sardonic invitation. “Put it in me, already. I’ve had four fingers tonight and you’re hardly the biggest cock I’ve taken.”

That might actually be another lie, but the human asshole is elastic and Gen more than most. Senku huffs, leans over, and starts to press himself in.

It’s a lot. It hurts just enough to help Gen keep his head where the look on Senku’s face might otherwise make him lose it again. He’s staring down at his cock and Gen’s asshole, deep concentration overlaid by something breathless.

When the head of his cock pops in, he closes his eyes. “O-oh.”

Gen doesn’t care to suppress a cackling laugh.

“Congratulations, Senku-chan! You’re a man now.”

Senku tries to look unimpressed but he’s got the pinched, closed-off expression of a man trying very hard not to come. Gen is beside himself with delight.

“O-oh! How does it feel, Senku-chan? Is it as good as you expected? Is it better? Do you think your father would be proud? Thank you for this flower, I will treasure it always!” It’s the furthest thing from sexy he can think of, but he doesn’t want it over so soon. Senku put him through the mental and emotional wringer for his orgasm. He wants some of his own back. And he has wanted Senku’s big dick for a minute. It would be a shame not to get as much of it as he can.

He almost regrets that this will be Senku’s first time, but candles and romance would be as lost on Ishigami Senku as they would be on Asagiri Gen.

Gen’s inane chatter has the completely intended effect of distracting Senku enough that he’s no longer fighting back from the edge. He looks less like he’s in pain, tipping towards exasperated. It’s time. Gen wraps his legs around Senku and pulls him forward.

“All the way in. I’ve been ready.” He gives a little theatrical eye roll and Senku laughs and pushes in.

He pushes in for a very long time. Gen gives silent thanks that he’s already come once on Senku’s bony fingers; it’s been thirty-seven hundred years since the last time he got fucked and Senku’s frankly absurd cock has him feeling every century.

When Senku’s finally fully inside him, Gen flicks his cheek to get his attention before he can move. “Give me a moment. You’re—” he can’t believe he’s going to have to say this out loud, but here they are. “You’re big. I need a minute to adjust. I’m very relaxed because I know how to do this and you gave me a lot of fingers and I’ve just come, but with anyone else you’ll need to go very slowly and be very careful.” It really can't be overstated. Senku is just too big to top without a lot of prep and an overabundance of caution.

Senku looks blank. And then he says, impossibly, beautifully, “I don’t want to, with anyone else.”

He's not saying it to be romantic. It’s a bare statement of fact, one that Gen could have inferred from past practice: he hasn’t with anyone else, despite numerous opportunities; he has with Gen twice now. All the same, Gen's cheeks heat and he looks away.

“Okay, move now, Senku-chan.” He braces himself for some enthusiastic but unskilled humping. What he gets is a feeling little thrust and Senku’s eyes on his face.

“Hmm,” Senku says, then tries it again.

Bends a little. Tries it again.

Lifts Gen’s knee into his lap. Again. Puts it over his shoulder. Again.

“Ah!” His dick bumps up against Gen's oversensitive prostate in just the right way. As raw as he is, it feels good.

Senku’s smile sharpens. “There?” He does it again. The sound it punches out of Gen is answer enough.

“Is it good?” It is. He’s sensitive, but he likes it. “Do you want to come again?”

The idea is ludicrous. He just came his fucking brains out not twenty minutes ago. He bats his eyes. “You’re welcome to try, Senku-cha— ah!”

“I think we can get you there.” Unshakeable confidence.

“I'm not even— hard!” Although, now that he’s thinking about it, his dick’s starting to get interested.

“Give it . . .” Senku pauses and squints. "Six minutes.”

He can't bring himself to doubt it. Senku’s got more than enough data to make a pretty good guess, and it tracks with what Gen knows about himself and his refractory period.

Senku sets into a dirty, methodical grind, and sure enough, Gen’s hard again soon. God fucking damn him and his knowing. He kind of wants not to come again, just to prove to Senku . . . something. That he doesn’t know Gen’s body as well as he obviously does. That he hasn’t mastered sex right out the gate. Which he clearly has.

Gen is fucked.

Well, if it can’t be helped — might as well lie back and enjoy it, right? He grabs the legs of the shelf above his head, breathes deep, and holds on. They’re not going hard, but it helps to have something to hang onto. Senku is a lot; not just his size, how he feels in Gen, but his . . . everything.

He doesn’t deviate much from his groove, now that he’s in it. It’s strangely consistent. He hits Gen’s prostate without fail, at a fixed rhythm. He can only do so much, Gen realizes; knowledge can only take Senku so far in this without experience. But what he can do, he’s going to try to do well. And isn’t that Senku all over?

He lets himself look at Senku as much as he likes, at his beautiful geometry and the curve of his mouth. He’s somewhere Gen never thought he’d see him go, a concentration beyond thought.

“Harder,” Gen tells him. Then: “Faster, too, if you think it won’t make you come.” He means to say it playfully but it comes out breathless and too sincere.

Senku nods. He’s panting, mouth open. Gen can see his tongue.

He wants it. “Kiss me.”

Senku stills, bends down to Gen. He can’t thrust and kiss at the same time so Gen moves for him, arching his back up off the floor to meet him. He kisses Senku and Senku can barely kiss back. His forehead is damp against Gen’s.

Senku overbalances and pitches forward onto him, elbows smacking the floor. Their chests press together. Senku’s nose is in his neck. He’s got Senku’s hair in his mouth. He doesn’t care. It’s the closest they’ve ever been. He grasps the back of Senku’s arms so Senku will pick up his head. His face is— he looks wrecked. Gen’s breath is a sob.

He expects to feel pride in himself for making it good, for making Senku lose his shit like this. What he feels instead is overcome.

He’s scared. “Harder,” he says. “Harder, Senku— fuck—!” Senku takes the direction, gives it to him the way he’s asked for, the way he’s begging for. His mouth is set and he’s breathing like he’s run a marathon. He meets Gen’s eyes, and Gen goes liquid and warm.

He cannot fucking handle how Senku is looking at him. “Harder, god damn it!” He can barely get the words out. Senku shakes his head, sweat flying off the ends of his hair to land on Gen’s chest, but he puts his head down and picks up his pace. Gen feels bruised. Gen feels wide open. Gen wants to fucking scream but he thinks he’s going to come instead.

“Touch yourself. I can’t last, Gen, touch yourself.” Yeah, okay. He can do that. His fist flies over his dick and he’s coming, and it barely feels good at all because it’s too fucking much but it also feels fucking amazing. He’s coming, he’s coming, he’s saying it over and over and Senku gives a relieved gasp then groans and shudders to a stop because he must have come too.

For a moment they linger where they finished, chests heaving, Senku's sweat cooling on them both. Finally Senku groans. He slides out of Gen and tips sideways, collapses to the floor beside him.

Gen’s spent more time staring at the ceiling of the lab tonight than he has, well, ever. He’s afraid to look at Senku, who is quiet and still beside him. He feels like he should say something but he can’t think what. The jizz on his belly is starting to itch, but he doesn't move to scratch at it.

Senku’s cum starts to drip out of him onto the lab floor. Gross. He’s making a mess. Thoughtlessly he reaches down to touch his sore, loosened asshole. There's no stopping it. Senku’s just too big.

For all the problems of the modern world, at least there’d been— “Condoms.”

“Hmm?”

Gen stretches his messy hand out as if to show him. He’s not sure if Senku sees or not because he’s not looking.

“No condoms in the stone world, Senku-chan. I'm making a mess.” And then, with a playful heat, “You made a mess in me.” He chances a glance over but he can't tell if Senku’s still red from his exertions or if he’s blushing a little.

“Early condoms were made of sheepskin.” Senku finally croaks. “Intestine, maybe bladder. We've already got sulfur and lye.”

“You're gonna need a pretty big sheep.”

Senku laughs, and after a beat Gen laughs too, a little hysterically. Laughing, stretched on the floor of the lab, sweaty and filthy. It’s disgusting. It’s wonderful.

Senku takes Gen’s messy hand.

As he lays there, Senku’s hand in his, for the first time in a long time Asagiri Gen smiles a very small, very real smile.