Prompt fill: Kakashi fucking Naruto against the wall with his hand over his mouth (to keep in those noises), when- wait! An accidental voyeur (who gets a good view of that glistening muscled back and flexing ass) appears- Naruto notices and his eyes widen- too late, literal eye rolling orgasm and arching and back clawing. Accidental Voyeur is jealous.

Iruka didn't quite understand his feelings right now. He didn't know what he wanted.

The way that Kakashi was fucking Naruto was powerful, snapping thrusts that seemed to twist Naruto up from the inside and smooth him out all over again. Kakashi was tall too, holding Naruto up as if it was nothing, as if the boy (teenager, really) weighed nothing, pinned high up against the wall and pleading for it.

"Kakashi-sensei," Naruto rasped quietly, as if he had no air, "I can't, I can't-" can't handle it? Can't hold his orgasm off? Can't breathe?

That's what Iruka felt like, like he'd walked into an alternate dimension that made Naruto into this lithe, soft creature of wetness and sound and Kakashi into a hard wall of muscle and scars, shining pale in the moonlight. Hell, they weren't even in the privacy of their room, Iruka knew for certain that Naruto's apartment was several buildings away to the left, what made the two drop drawers in the quiet Konoha alleyways and, and, and fuck like animals?!

Even thinking the word fuck made Iruka blush and squirm, shocked to find himself breathing heavier and riveted by the sight.

A strangled gasp made Iruka blink through his daze, Kakashi had thrown his head back and shuffled Naruto lower, and so the boy seemed to hang there between solid muscle and brick wall, legs spread obscenely wide. His thrusts were hitting deeper, pulling beautiful noises out of the boy and clearly getting to Kakashi, because biceps were flexing and his back glistened in the blue light. Blue eyes were hazy, looking into the shadows where Iruka was hidden, and the teacher thanked every lucky star that Naruto hadn't seen him. Iruka couldn't see where the two were connected, but he could hear it.

Every flex of Kakashi's back, thrust of his perfect ass, half hidden by the jounin's sloppily shoved down pants, and a slick thud seemed to resonate in Iruka's gut. He half thought he imagined it, that meaty grind into a warm hole, a tight hole, by the way Kakashi was growling now, low vibrating growls that sounded in perfect counter point to Naruto's half slurred begs. They were fucking so hard, and it was a wonder to see Kakashi's strength, holding Naruto there and still filling him so good.

"Oh my god, please, I'm going to, right there, right- There, Kakash—" silenced suddenly by a gloved hand, since the blonde was getting loud, Naruto started flailing a bit, his arms scrambling to hold onto something. He was going to come.

Iruka was shamed by the hardness in his pants, but holy, holy fuck, this was hot and heavy, and Iruka had never had sexual relations like it. A hand wavered, travelling to his crotch without bidding and the first brush of fingers made him bite his lip, eyes glued to the scene. He was a slow lover, a gentle one, and he wondered, darkly, hidden in the shadows and not feeling like himself, if Naruto would scream into his hand like that, or if Kakashi would tremble that way if Iruka was his hole. Maybe he was jealous of the way that Kakashi fucked his student, his sixteen year old student like it was his last day on earth, like Naruto as there just for him, like with every thrust of his cock he took more and more of the blonde for himself.

Before he could slip away unnoticed, honestly, it was a shock that Kakashi hadn't noticed him yet but Iruka figured he wouldn't pay attention to anything except the way Naruto was coming around his cock, milking him like that, Iruka realized that the haze in Naruto's eyes was growing incredulous, confused, with realization filtering in.

But even Naruto couldn't deny the power of his own orgasm, Iruka felt blown back by the boys wail even though Kakashi had a strong hand across his mouth, practically suffocating him. Naruto jackknifed, clawing at Kakashi, shaking and drawing Kakashi's orgasm alongside his own. Iruka noted, somewhat numb but overly sensitive – his fingertips on his cock were frozen because he was sure if he moved he'd come right then and there – that Hatake Kakashi bit down when he came, moaning through tan flesh his release, coming so much that it gushed when he kept thrusting, falling to the dirty alleyway floor.

"Iruka-sensei?" Naruto gasped with his first breath after orgasm, still twitching and trembling around Kakashi, clenching fists into Kakashi's hair. His mouth was wet, red, and tempting, and his voice, breathing around ecstasy like that, Iruka would never forget.

Iruka shunshined out of there before he could be identified by Hatake Kakashi as a voyeur, hoping futilely that Naruto would forget it forget it please, because he never would. Three minutes later Iruka banged into his empty home, locked his door and fell to his knees, his cock already out and weeping. He thrust up into his hand, sucking in air through his teeth and coming right there at his front door, wishing desperately that he could be the kind of man to fuck like that in an alleyway.