He closes the door softly behind him. He's not sure this is a good idea. Not at all. In fact, this is the worst idea he's ever had. Right after taking a midnight stroll through the woods to find a corpse. Nothing will ever top that, really.
He can feel every single muscle in his body, clenched tight, waiting; he has an hyper awareness of the rise and fall of his chest, of how the air slips through his mouth, moist and warm, of the thin sheen of sweat coating his nose, his nape, the tips of his fingers.
He can feel everything, see everything, hear everything. The wood behind him, the towels hanging a few feet away, the voices of both his father and Derek, making small talk in the kitchen.
Everything, it feels akin to being connected through a cord in his navel to the outer world. A little like burning, a little like being ignited from the insides by a little spark. Mostly like painful arousal.
He feels dirty, feels guilty (his father's out there, just barely out of hearing range). Feels...
Feels sexy. A little.
"Have you ever heard about payback. Derek? You probably have. Well," He says, voice already a wavering wreck. His hand goes for his clothed erection, heel pressing down. He sighs, then moans at the sensation of the zipper digging in. "Payback's a bitch, Derek."
"You're probably going to kill me for this," he gasps, the fingers of his left hand (the one not cupping his dick) grazing the dark trail of hair above his jeans. "but it's going to be so, so worthy. Having you sit through a polite conversation with my dad while knowing that I'm here," he bites his lip briefly as he directs his right hand towards his belt buckle. "touching myself and thinking of you."
Once his belt's unbuckled, and his jeans are undone and shoved down his legs along with his underwear (his wet, sticky underwear that probably reeks of his want and that he'll keep on after this just to have Derek on edge for the rest of the day) his right hand goes to grab his dick, not having much time to drag this for too long.
The first tug goes to his gut with the ferocity of a punch, and he lets out the littlest, lowest, sluttiest 'oh God' in the known history of mankind. Or pornkind. This is way better than doing it alone in his bedroom at night, because he knows he's got Derek's attention, knows that he's out there, having no other choice but to listen to him get off; has to listen to his every sound, the slick skin on skin, his broken voice, his racing heart wanting to burst out of his ribcage through violent beats.
"Oh, God, Derek ." he bites out, fighting to keep his voice down, one hand jerking tightly and twisting slightly on the way up and using every single trick he knows; the other alternating between softly handling his balls and rubbing his perineum (oh, google, wonderful blessing, always ready to teach the things school sees fit to leave out in fits of prudishness) with the side of his thumb. "You have no idea how good this feels, dude. No idea. This is the best me-time I've ever had. Ever."
He licks his lips, moving his left hand to have it trail experimentally along the cleft of his ass, breath hitching at the sensation.
"This would only be better if you were here." It's true. It's such an immense truth, it makes his thighs tremble and thrills run up his spine and his dick throb even more with the sweet ache of need. His left hand parts his ass cheeks, then. His right hand almost stops its ministrations but he takes a deep gulping breath and keeps going, feeling closer and closer to his orgasm by the second.
"It would be so good." He almost whipers, one finger reaching his asshole and putting a little pressure there. "You probably wouldn't fuck me, though, with my dad this close." The finger prods, and he keens. It's kind of a testament to how determined he is to getting off and doing it faster than fast that the brief thought about his father doesn't make him any less incredibly aroused than he is, doesn't come even close to softening his dick. His right hand goes quicker and quicker, thumb rubbing the slit mercilessly, dragging with it the moisture of his pre-seminal fluid everywhere.
He takes his hand from its place teasing his ass and brings it up to his mouth, sucking his index and midde fingers in, coating them with his saliva (before it would've grossed him out, sucking on a finger that'd previously been that chummy with his butt. My, how sex changes people's perceptions of what's gross and what's hot and whatnot.), only to get them back to between his ass cheeks when he's done, letting them tease a little more, getting him used to the burning sensation of the tips applying pressure.
"I'm so fuc- I'm so close. So, so close." His chuckle is a pitiful mix between a moan and a sob. "It would be embarrassing if I wasn't trying for it."
The tips of his fingers go in and his right hand flies off his dick to scratch at the door. "Oh, oh, oh. This is so. Oh God." When he's gotten mostly used to the feel his hand relaxes a bit, but he leaves it there for the extra support. In case his clumsiness decides to make a surprise appearance and he falls and breaks his everything.
"This is so good. I can't believe I'm this sensitive here." He moves his fingers a little, getting them deeper inside and curling them, and feels his whole body shudder; he sees the way his dick almost jumps, seeping a few more dropplets of pre-come. "Your dick feels so good inside me, man. So, so, so good. That's like the only thing that I prefer to having you fuck my thighs until you come all over my legs."
That. Oh, that. That's a nice thought, there. A lovely, lovely image. His fingers go out and thrust in again, ruthless, painful and so good. And between the rough pleasure, and the knowledge that Derek's out there listening in on this, and the memory of Derek's hot and sticky come dripping down the inside of his thighs to finally land on the mattress, he's done. Coming so hard that he's pretty convinced that he's dying. So hard that the pain of hitting his head against the door barely registers.
"Stiles, you little shit."
Not hard enough, though, to not be able to grin drowsily at Derek's pained growl just a door a way and mutter, " Payback, Derek. Such a wonderful bitch."
'Kinda like Lydia.' He thinks. He's got enough brain cells to know how unwell that'd fair with Derek (who's got so many issues, and such a low self esteem that it hurts, and is the most possessive person he's met), however, so he keeps his mind shut as Derek goes back to his dad and makes up a story about Stiles and intestinal viruses and diarrhea so horrible and cringe worthy that it has the sheriff knocking on the bathroom door to ask in his most parental voice if he needs to be taken to the hospital.
He kinda deserves it.
(That's Derek for you, A+ in exacting revenge.)