Zenitsu hears it coming just a second before Tanjiro smells it, that particular tipping point where something--god only knows what, considering who it is--flips Inosuke's switch from Fight to Fuck. He's holding out his hand for the straps of Nezuko's box before Tanjiro can ask, red-faced, red eyes a touch too bright for either of them to pretend he isn't at least a little excited by what's to come.
And well, Zenitsu can't say he minds it so much either. The first time this happened, Zenitsu was horrified, scandalized. But he's a changed man now, the sort of man who sees his two best friends gearing up to fuck and takes a seat to watch.
(He kindly puts Nezuko's box behind a tree-- she's sleeping, luckily for everyone involved, but still--it's the least he can do to shield her from their collective debauchery.)
By the time Zenitsu settles himself down, leaning against the side of the tree facing his friends, Inosuke's got his boar head off, which Zenitsu knows means he's feeling extra needy today (and of course Zenitsu has picked up on Inosuke's sex quirks--how could he not, when he’s pretty sure he’s borne witness to nearly all of Inosuke’s sexual exploits over the past year, to some degree or another). He's also opened Tanjiro's pants up, his hands moving under the fabric, no doubt working Tanjiro's length to full hardness.
Zenitsu hears every one of Tanjiro's increasingly labored breaths as if they're right next to each other, every murmured "Oh there, just like that" as if Tanjiro's whispering into his ear. Inosuke growls softly in response, keeps his nose buried in Tanjiro's neck, huffing in the scent of his rising sweat in loud, heavy breathes as he savors having Tanjiro close, feeling his heat, the solid weight of Tanjiro's body against his own.
Then Inosuke looks up and Zenitsu’s gaze catches on bright green eyes, holds Inosuke’s stare as he pets himself over his trousers.
Inosuke clicks his tongue, "Oi, Monjiro, see that?" He asks Tanjiro, grabbing his flushed face by the jaw, making him meet Zenitsu's gaze as well, "He doesn't even have his dick out yet! Think he's feeling shy?"
Tanjiro's gaze rakes over him like hot coals, taking in the violent pink of his cheeks, the slow rhythm of his hand moving over his crotch. He can probably smell the lusty interest oozing out of Zenitsu's pores, just like Zenitsu can hear Tanjiro's blood pounding out an increasingly heated rhythm.
No, neither of them are feeling especially shy.
All the same, Tanjiro licks his lips and says, "Maybe."
Inosuke barks out a laugh, and hardly a second later he's got Tanjiro's pants and underthings down around his ankles, exposing his hard cock right there on the side of the road.
Zenitsu would never say it, but objectively Tanjiro has a cute dick. It's perfectly in proportion to the rest of him, rosy with want, and curves up just a little. And while Zenitsu rarely gets to see it as clearly as he sees Tanjiro’s, Inosuke's not exactly lacking in that department either. His dick is as unfairly pretty as his face, and unfairly large too. Between the two of them, Zenitsu's had more than one fantasy of touching, teasing and tasting them--he defies anyone who has ever seen their dicks not to!--he’s just never quite been sure if he’s allowed.
In a daze, Zenitsu drags his eyes slowly up from Tanjiro's cock, traveling over the dark curling hair at its base, his chiseled stomach and scarred chest, back up to his flushed face, just in time to see Inosuke turn his face to the side again to capture his mouth in a deep, sloppy kiss.
God, the sounds--the wet slide of their tongues, the building whimper in Tanjiro's throat, the way Inosuke hums, so satisfied with himself as he ravages Tanjiro's mouth. And well, they're making such an effort, the least Zenitsu can do is get his own cock out of his pants.
Zenitsu sighs when he gets himself free, starts himself a slow, indulgent pace. He likes to try and come at the same time they do--makes the whole thing less awkward all around he's found, if they’re all done at about the same time.
"Nnh, Ganpachiro, where's that weird stuff of yours?" Inosuke asks, his hips working themselves against Tanjiro's exposed backside in a steady rhythm. Zenitsu can't see, but he's pretty sure from the sound of it that Inosuke's got his own trousers opened up just enough for his bare cock to be out, no doubt rutting into the cleft of Tanjiro's ass like the beast he is.
Tanjiro catches Zenitsu's gaze again at the question, and Zenitsu remembers, vividly, the way Tanjiro had furtively pressed a little vial of oil into Zenitsu's hand not too long back, and had begged with his eyes for Zenitsu to hold onto it for him.
Tanjiro's hand had been so warm against his, the little vial so cool.
Zenitsu's not usually one to struggle finding his words, but this time he has to swallow thickly before saying, "I have it."
He has to stop what he's doing to fish the bottle out of his clothes but he does it quickly enough, his hand shaking faintly as he holds it out to Inosuke. Inosuke looks down at the vial with interest, then grins up at Zenitsu, his smile wild, all teeth as he declares, "Good job, Zezizu! As a reward, you can get Tanjiro ready!"
Zenitsu chokes on his own tongue. He looks back to Tanjiro for a moment, sees his embarrassment. But for all that the statement flustered him, Zenitsu can hear that it didn't surprise him.
"I really MUST insist," Zenitsu begins, "that you give ME some prior--oi--!"
"Put up or shut up, Monitsu!" Inosuke complains, and bodily shoves Tanjiro forward, so he stumbles over his feet and has no choice but to catch himself on the very tree Zenitsu is leaned up against.
Like this, Tanjiro is close enough to kiss. Hell Inosuke too, with effort.
"I--I--!! T-this is really quite unexpected!" Zenitsu can't help but stammer out, even as Inosuke reaches around Tanjiro to take the vial from his hand and dump a slick stream of it out over his fingers. Zenitsu can hardly process the way Inosuke's rough hand spreads the oil over his fingers, his mouth the only thing still working, completely independent of his brain as he says, "While I assure you I know exactly what I'm doing, and that this is--well, indecent! You two have made me into a complete deviant! But--this is the sort of thing people really ought to talk about with a clear head--"
"GAH! You talk too much!" Inosuke interjects, yanking Zenitsu forward by his oiled hand, bringing it back around to Tanjiro's rear. Warmth, the space between their bodies is singing with warmth. "Last chance--do it or don't!"
Zenitsu does it.
"Oh, oh--" The first dip of Zenitsu's fingers into Tanjiro's body is accompanied by Tanjiro's sweet, surprised little gasp, the sight of his eyelids fluttering like Zenitsu's seen so many times before, but never from so close, never because of him. Now that he's started, he can't stop, so curious and hot for the sensation of Tanjiro's body so tight around his fingers, opening for him. The angle is awkward, but not as awkward as when Zenitsu does this to himself, on his rare nights alone, so he can ignore the slight twinge in his wrist easily, more focused on the softness of Tanjiro’s insides, the pleased hum of his body.
Just a few minutes after Zenitsu starts, he feels Inosuke slide a finger in next to his, the two of them working in tandem to get Tanjiro loose and ready.
"Keep going, keep going, please!" Tanjiro pants out, the open need in his voice enough to make Zenitsu's legs quake. Zenitsu wants desperately to kiss him, to kiss either of them.
He's just not sure if he can--if this moment is part of some sex game between them all or an invitation into something deeper. He wonders if their sounds will fall out of harmony if he tries, or if they'd get hotter, sweeter. He doesn't have the courage to find out.
Inosuke suffers no such conflict. He grabs Zenitsu by the hair with his free hand and pulls him in for a filthy open-mouthed kiss over Tanjiro's shoulder, licking into his mouth and dominating it like Zenitsu's seen him do to Tanjiro a hundred times, like he’s always wanted to experience for himself.
The kiss is overpowering. Inosuke's lips are softer than any girl's, his tongue domineering, his raw passion for the moment breathtaking. When he lets Zenitsu go, he falls back against the tree in shock, his fingers slipping out of Tanjiro, his knees giving out from under him so he goes sliding down to the forest floor, until he's at eye-level with Tanjiro's beautiful, dripping cock.
Overwhelming. Too overwhelming.
"Zenitsu, are you--?" Tanjiro asks, and even though it's written all over his face that he's halfway to bliss, the concern in his voice is still genuine.
Zenitsu looks up at his companions again, sees their bright, desirous eyes on him, sees the dripping of Tanjiro's cock and the shadow of Inosuke lining up with the heat Zenitsu had been knuckles-deep in moments ago.
Zenitsu spreads his knees, takes his exposed cock in hand as his answer.
Inosuke's hips snap forward, Tanjiro's eyes snap shut.
"You feel extra good today!" Inosuke groans out, peering down at Zenitsu from over Tanjiro's shoulder, and--both of them, both of them are watching, eyes hungry, as hungry as Zenitsu is to watch them.
"Is it because Zenitsu helped?" Inosuke asks, "Do you want him to always touch you too?"
"Yes!" Tanjiro shouts, immediately, like if he says it with enough conviction Zenitsu will stick his fingers into him right along with Inosuke's cock.
"Good!" Inosuke says, his hips moving faster, one hand reaching around to jerk Tanjiro's cock again, like a reward for a right answer. "I think next time he should do a lot more with us both, too!"
"Please, please," Tanjiro chants, polite even on the verge of orgasm, like he always, always is. His cock is leaking right in Zenitsu's face, he can probably feel Zenitsu's breath on his cockhead. "Inosuke, Zenitsu--I--I--"
Zenitsu matches the speed of his hand moving over his own cock to Inosuke's moving over Tanjiro's, his breath coming in harsh pants as he tries to hold himself back from orgasm for just one, two, three seconds more, then--
Tanjiro cries out, near-silent, and comes hard across Zenitsu's face. The shock of it, of hot, filthy liquid hitting his skin, dribbling down his cheeks, sets Zenitsu off hard, and it's with a surprised gasp that he comes at their feet, his whole body shuddering with the promise of next time, of touching them both, of doing more than just watching.
Inosuke must come soon after because he drops a spent Tanjiro into Zenitsu's lap, where the weary boy fusses over Zenitsu's face, trying to wipe him clean, but mostly just succeeding in rubbing it into his skin. Inosuke seems to understand that to be the goal, because when he joins them on the ground that's what he starts doing.
"Excuse me! That is not a face cream it is ejaculate!" Zenitsu complains, "Be good like Tanjiro and kindly remove it! And take responsibility for aiming at my face! Heathen! Villain!!"
Tanjiro goes bright red, tries to stammer out an apology, but Inosuke laughs right over him.
"Of course I did, your ours now!" He says, as if that explanation makes sense. "Officially!"
"What Inosuke means," Tanjiro says with a fond sigh and a soft smile, "is that we, we care about you very much, and we weren't sure at first how you'd feel about being with men, but um, given the uh, circumstances, we thought--"
"Be our lover!" Inosuke declares, "You basically are already!"
"Well!" Zenitsu says, filled to bursting with flusterment and fluttery feelings, hands coming up to hide his burning cheeks, "Well!! Yes, that's--yes, it's the--the proper thing, making an honest man of me finally! Really--"
Tanjiro kisses him, sweet and tender, a soft, lingering press of lips. Inosuke kisses him immediately after, his savage smile fitting just right against Zenitsu's mouth. Zenitsu spends the rest of the afternoon getting showered in kisses and asking questions in between about how they'll work, how they want to work.
And of course, if he can still watch-- sometimes, at least.