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Derek is asleep when Stiles gets home. Like, passed the fuck out, sleep-of-the-dead asleep. Stiles would be worried—Derek never sleeps that soundly—but it's been a long fucking week.
The window is open in a vain attempt to invite a breeze, and there's a trail of Derek's clothing leading from the window to the bed. An errant black sock dangles from his left foot, but he is otherwise stunningly, deliciously naked.
Stiles drops his backpack and toes the door shut behind him. He absolutely does not do a silent dance-shimmy thing because he's excited Derek's left himself open and vulnerable like this.
Peeling off sweaty layers of clothing, Stiles curses the heat wave that's been plaguing Beacon Hills for the last week. Then again, he wouldn't have this spectacular view of Derek's ass without it. If he still had his pants on, he'd be tempted to find a quarter to bounce off of it.
He should probably lie down and go to sleep, but he hasn't had much opportunity for ogling as he and Derek are usually busy researching, fighting, or fucking. The fucking is awesome—you'll never hear Stiles complain about it—but he is inordinately pleased by this opportunity to just look. Hell, he's also curious as fuck to see what he might be able to get away with.
Derek is absolutely fucking gorgeous laid out for him like this. The moonlight casts delicate shadows across his back, making it look somehow longer and leaner than usual. He has one knee conveniently bent, opening his ass cheeks slightly.
It's mouthwatering, this little glimpse of Derek.
After a moment's hesitation, he kneels by the bed, unwilling to risk waking Derek by climbing on it. (Yes, sticking a finger in his butt is probably going to wake him up, but Stiles isn't really working with logic right now.)
Placing a tentative hand on Derek's buttcheek, he pulls it to the side just a smidge, just to open things up a little more. He sucks two fingers into his mouth, getting them nice and wet, and then runs one gently over Derek's asshole. He rubs it a bit, then wets his finger some more.
On the next pass, he puts a little pressure behind it and his finger sinks in to the first knuckle. Derek grunts, but remains asleep. Stiles, on the other hand, swears he can feel his heartbeat in his cock. He is rock hard and ready to go. He moves his finger, a tiny push-pull movement, but he's going to have to get Derek a lot wetter to get any more in his ass.
He leans forward and licks the rim of Derek's ass where it's stretched around his finger. He quickly gets a rhythm going—lick, push, pull, lick, push, wiggle—and before he realizes it, his whole index finger is buried in Derek's ass.
"Jesus, fuck, that's hot."
"If you stop now, I will end you," Derek grunts.
"Holy fuck! You scared the shit out of me. Why didn't you tell me you were awake?"
"Why would I? You were doing fine. Get back to it," Derek says with a roll of his hips.
Stiles is fine with this, so he wets his fingers again and goes in with two this time, breaching Derek a little quicker.
Derek arches his back, pushing his ass up into Stiles’ fingers, and moans into the pillows. Stiles pumps his fingers a few times before he needs more saliva. Derek must feel the drag of Stiles fingers on his insides because he reaches back with both hands and pulls his ass cheeks apart. It's obviously an invitation for Stiles to dive in again. He accepts.
Licking around Derek's rim, Stiles wets his fingers, and spreads them apart ever so slightly, making room for his tongue to get inside. The groan that follows sounds like it's ripped from Derek's chest.
Fuck, he's never seen Derek like this: rutting up into his hand, down onto the bed, moaning incoherently. It's the hottest thing Stiles has ever seen, and there's no way he can ignore himself anymore. He licks his palm and takes his own cock in hand, jacking himself to the same rhythm he's using on Derek.
He curls his fingers downward, moving them in tiny thrusts aimed for Derek's prostate. Derek nearly fucking howls when he hits it, hands falling to the bed for more leverage to push his ass up on Stiles fingers.
"C'mon," he whispers, encouraging Derek to let go. He plants a hand on Derek's ass and climbs onto the bed, straddling Derek's thighs. Pushing him back down, he rubs two fingers against Derek's prostate. Derek freezes and comes with a whine at the back of his throat, trembling and twitching.
Stiles isn't about to waste the opportunity before him, and starts jacking his cock over Derek's clenching hole. He won't take long, not after watching Derek fall apart on his fingers like that. His balls are already pulled up tight and he can feel the orgasm building in the pit of his stomach as he imagines what it will be like to get his dick in there. He presses the tip against Derek's asshole. Derek clenches, and that tiny movement is enough to overwhelm Stiles’ senses and he is suddenly, blindingly, coming all over Derek's ass.
"Fuck," he cries, and collapses on top of Derek, dick nestled comfortably in between Derek's cheeks.
It takes his heart a minute to calm down, another to get his breathing under control and roll off of Derek.
Who is asleep again.
"Stupid, gorgeous jerk," Stiles mutters, and flops onto his back, utterly exhausted.