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Stiles is a young 18-year-old guy, so as all 18-year-old guys do, he's currently masturbating on his bed because the house is finally empty.
Well, maybe not all 18-year-old guys kneel completely naked on their bed facing a giant rectangular mirror at the head of it, watching themselves play with their nipple with one hand and finger their ass with the other, but whatever. It's not weird.
He moans as he watches himself, feeling like a narcissist but also kinda getting off on that as well, shoving another finger in alongside the one already in there pressing against his prostate. He twists his nipple and arches his back. God, it just feels so good, why the fuck does it feel so good?
He's been doing this thing for months, so he doesn't even need to touch his cock even slightly to cum anymore, just the stimulation from his prostate enough to reach orgasm. But right now, he has all the time to thoroughly enjoy himself, dad finally gone to attend to some petty crime in the dead of night. So he doesn't let himself cum, just keeps getting himself off till he's panting and delirious with pleasure before stilling, catching shaky breaths before doing it all over again.
Stiles has long forgotten how long he's been doing this. It could be only ten minutes, it could be forty, he has no fucking clue. He just keeps going because it feels fucking amazing.
He starts moaning out a name, at first quiet under his breath, but steadily rising as he approaches climax, adding yet another finger inside of himself and starting to grind his hips down onto his hand.
"D-Derek, Derek, fuck, Derek!" he shouts.
He closes his eyes, stroking his prostate gland harder as he feels thin semen leak onto his stomach, a steady tiny river now. "Ah, Derek, fuck me, please! O-oh fuck!" He twists his hard nipple and cups his flat breast, leans forward as he feels his legs start to shake from being so close to orgasm.
God, he can almost fucking see him, likes he's right there, right in front of him. His built muscles, tall, solid figure, hands big enough to pin him down or fuck him up if he wanted, which Stiles might actually be okay with in the right mood. Such a fucking handsome face, one he could touch literally all day and find new things to admire about. And that fucking cock, veiny and tinged red and so fucking gigantic, just long enough to fit in the back of his throat, just wide enough to stretch his hole out to its maximum and hurt him the slightest bit when he knots. Oh God, and knotting, fuck, sometimes it was worth all the weird shit Derek did and said just for that damned bulbous thing which would slam into him and fuck him full of endless amounts of his hot sperm.
Dear lord, it had been—how long had it been since Derek last fucked him—three days! Three whole fucking days, and that wasn't even actual anal, that would be more like an entire fucking week ago! Fuck, Stiles would let Derek pin him down and use him as a fucking cocksheath, a fleshlight, right fucking now if it just meant he could have him inside of him, holy fucking shit–
And then Stiles stops, panting heavily with his eyes still closed, keeping his hands still on his body and fingers still buried inside of his hole, just not willing himself to move them at all. He doesn't want to cum, not yet, it will ruin the potential hours of fun he could have.
And besides, at this point, after doing this whole thing for months, his body seems to have grown used to being edged so near orgasm and then coming down from it. It's almost preferable to actually cumming at this point, somehow.
So he slowly comes back to reality, opening his eyes blearily as he waits for the pulsing of near-climax to go down so he can go for yet another round, dopamine flooding his brain as he breathes through slightly parted lips, smiling to himself.
And then he about jumps out of his fucking skin, ripping his fingers out of himself and flipping one-eighty to face the fucking nightmare he'd seen for a nanosecond in the mirror, nearly screaming as he falls on his ass.
The tall figure behind him—in front of him now—just smiles a little.
Oh, thank fuck, it's just Derek.
Wait, it's Derek?
"How the fuck?" Stiles says succinctly.
Derek rolls his eyes, shuts the window behind him. " You left your fucking window open. And there's no screen, just a curtain, so it was pretty simple."
Stiles tries to cover his bare body with his hands, it of course not doing much at all. "Dude, you can't just barge in here! I'm fucking busy!"
His boyfriend grins. "I could tell."
"Well, go! Get outta here!" Stiles pouts, crossing his arms.
Derek just shifts his head, continues to stand right there. "Is that really what you want, though? Because it sounded like you wanted me to do something to you, that didn't actually involve me leaving at all."
Stiles blushes at the memory, suddenly seeming so shameful. Fuck, how long had Derek been watching him? For all he knows, it could have been ever since he first closed his eyes, maybe even before that, just waiting beneath his window. Fucking creep.
"Well, not anymore! You ruined it."
"Or did I?" Derek says, scenting the air with one mighty breath. "Because I can still definitely scent arousal on you. In fact, I think you're leaking again."
Stiles moves the blanket beneath him to cover his legs. "Like hell I am."
Derek starts to walk over to him, sniffing the whole way as his pupils dilate in the subtle lamplight of Stiles' room. "No, you definitely fucking are. It's unmistakable."
He puts one giant hand on Stiles' fleshy thigh, digging his nails into it as he easily moves it away from the other and over the covers despite Stiles' weak human muscles straining to keep himself decent. He grins at the sight of Stiles' little leaking cock, the organ visibly twitching under his eyes.
"You've been fucking teasing me for over half of a fucking hour now, Stiles. Don't you know what that does to a guy, let alone a guy who's a damn werewolf with hard mating instincts?"
Stiles leans back slightly, shifting his weight onto his elbows as he watches Derek just fucking stare at his dick. "I-I have no idea what you're talking about."
Derek looks up at him, squinting his eyes. "You don't? How the fuck wouldn't you, you know how sensitive my hearing and sense of smell are. Not only were you fucking leaking like a broken tap with your damn window open for all to smell, but you're moaning my name, for me to come fuck you. What the hell am I supposed to do about that?"
Stiles shakes his head, thighs twitching with arousal as he only drips more. "I didn't mean to, I just got lost in the pleasure… I-I forgot."
"Of course you did," Derek says as he lays his hand over Stiles' crotch, making him shiver, "because you're a fucking slut. And you were sitting here, moaning, shouting, for everyone to fucking hear."
"B-but it's my room…" Stiles murmurs.
Derek tuts. "With the window open? It's fucking cold outside."
"I-I got hot– hah!"
He's grabbed Stiles' cock with his hand, it fitting inside of his large grip easily, and starts pumping it mercilessly. Stiles' hips buck instinctively, and he cries out loudly, "No, no, no!"
"Hm?"
"D-don't touch my cock!"
Derek relents, surprised enough to stop. "What?"
Stiles pants, "Don't, don't touch it."
"Why?" He looks confused.
"Be-because I want…" Stiles looks down sheepishly, "I want to just cum from my ass. I've been training myself to do it."
After a second or two to mull it over, Derek laughs. "Really? Why didn't you fucking tell me before? That's awesome."
Stiles can't reply before he's doubled over, thighs spread to either side as a silhouette pins him down. It all happens in half a second at most—damn werewolf reflexes—and then Stiles can't speak yet again as a mouth presses to his, tongue instantly invading past his closed lips. Large fingers come down between his ass, circling his wet hole before pushing inside, two instantly fucking in and out of him.
"Ah," Derek says as he gives Stiles a chance to catch his damn breath, "I see why you don't want your cock touched."
"Wh-why?" Stiles asks, baffled.
Derek leans in and grins, twisting his wrist to probe his prostate as he whispers in his ear, "Because you wanna be fucked just like a girl, don't you, Stiles?"
He calls out, grabbing fistfuls of Derek's shirt as though for dear life. " No! " he whines.
"No? But why else would a guy want to be able to orgasm just from his ass, unless to be like a fucking girl with a pussy?"
Stiles clenches down on him at that last word, making him chuckle as he strokes his other hand over his soft whimpering lips. "You like that word, huh? Pussy?"
He adds another finger in, wide enough that they're about equal to four of Stiles', stretching his hole as wide as a cock. "Is that what this is? Not an asshole, but a cunt?" He points at Stiles' throbbing cock, laughing condescendingly. "And this isn't a cock, is it? No, it's not nearly big enough to be an actual man's cock, it must be just a little clit." Stiles feels real shame burn through him, even though he knows he isn't that little. Maybe a bit below average, but…
Stiles tries to squeeze his thighs together in embarrassment but only presses against the solid mass that is his boyfriend. "Oh, God," he sobs.
With one last thrust of his thick fingers into Stiles' pussy, Derek withdraws with a slimy squelch, moving back up and taking his wet fingers right above Stiles' face and making a show of licking them clean.
"Oh," Derek hums, "flavoured lube, Stilinski? Interesting."
A few drops of it spill onto Stiles' face, one into his mouth as the taste of his own lube used to fuck him coats his tongue. "Ch-cherry."
Derek presses one last kiss to Stiles' lips, inserting his tongue so he's sure Stiles gets another taste before moving down. "Fitting."
He slithers down Stiles' body until he reaches his hips. Now falling off the bed, he grows quickly irritated and so just grabs the human and practically tosses him upwards to the corner of the small bed to make room for his much larger body. Stiles hits the covers with an oof, but it's not long before his daze is cured by an expert tongue licking up his crack, quickly moving inside of his hole.
"Mmm," Derek says between licks and fucking his tongue in and out, "if only your pussy could always taste like cherries, I would never stop fucking eating it out."
Stiles groans, reaches down to rake his hands through Derek's hair and pull on it whenever he licks against his prostate, earning a growl. "Mmm, yeah," he moans, "l-lick my pussy, D-Derek..!" It feels weird to say it, but by the time he can think about it, it's already out in the air, too late to take back.
Derek shoves a few fingers in his cunt as he slides his tongue out, humming in pleasure himself as he drinks up the juices of the lubricant. "Yeah, you really are a slut, aren't you, Stiles? Saying shit like that, God, you're fucked."
Stiles shakes his head. "I-I'm fine. I'm not a s-slut."
Derek pulls him up roughly by the arm, popping some joint along the way—hopefully harmlessly—and Stiles finds himself on top of Derek's lap, now facing the mirror entirely.
"You're not a fucking slut? Then look at yourself," he grabs the nape of Stiles' neck, forcing him to turn perfectly straight to the mirror and stare at himself, "tell me you don't look like a fucking whore."
Stiles flushes completely, wide eyes taking in his unruly appearance: wet, messy, coc– clit leaking even as he notes the drying cum on his belly.
He manages to shake his head still, frowning.
Derek sighs. "Well, I guess I'll just have to show you that it's true."
Stiles is lifted up and then sat back down, but his eyes light up instantly as his cunt is stretched beyond belief, looking in the mirror to confirm that, yes, Derek just impaled him on his massive cock. It looks almost ridiculous, spreading his pussy wide at the very bottom, easily the width of two of his own dicks.
Derek grabs his sides and forces him up and down roughly, snapping his hips up and down as he sets a brutal pace which makes Stiles' whole body jiggle in the mirror, never giving him a break as buckets-full of pleasure wash over him, his toes curling and moans streaming from his mouth.
"God," Derek growls, "you even fucking moan exactly like a girl, don't you? God, your pussy's so fucking tight."
He reaches one hand down to Stiles' ass, spreading it apart so he can fuck him easier. "Do you see it now? What a fucking whore you are?"
Stiles refuses, shakes his head vehemently, no, he couldn't possibly be a worthless whore.
Tears begin to prick his eyes for some reason, probably from a mixture of pleasure bordering on overstimulation, pain of being fucked so hard with little time to adjust, and pure, intense shame of having to look at himself as he's taken from behind and enjoying it so fucking much."
A hand comes down upon his ass, pain registering in his brain. Derek just fucking spanked him. "Slut."
"No, no, no–"
Another smack, this time much harder, coming just as Derek bottoms out. Stiles watches through one eye drowning in tears as his own cock jumps at the combination.
"Fucking accept it, Stiles!"
When Stiles doesn't react, Derek keeps going, thrusting and hitting his bare ass until it's red, feeling his cock begin to expand at the base, a marker of his werewolf genes. He reaches his hands up to play with Stiles' tits, twisting and pulling at them until it hurts so much Stiles whines.
Stiles feels the drag of Derek's knot now pushing inside and outside of his stretched cunt, looks in the mirror to see what almost seems to be a completely different version of himself: mouth parted, drool running down his lips to his chin, tears streaming down his cheeks, nipples hard as diamonds as Derek's huge hands tug on them as though to milk him. His oversized clit bounces up and down lazily, pussy being hammered as something even fucking bigger moves inside of it, thighs shaking and twitching familiarly.
There's no use in denying it anymore.
"Ah, I'm a fucking slut!” Stiles screams.
His hands grip Derek's muscled thighs enough to bruise as he cums, squirting pathetic amounts of useless sperm onto the mirror in front of him. Derek fucks into him a couple more times, fast and fucking hard enough almost to worry about Stiles' bones cracking, before his knot finally pops in one last time, secured inside of Stiles' cunt. Derek's hands rack down to Stiles' hips, and when Stiles feels the sensation of piercing through his skin he watches as red begins to show from his breasts all the way to his sides, left by Derek's sharp, careless claws.
Derek pumps him full of cum, automatically moves his mouth down to Stiles' neck to suck and bite at it. For a minute Stiles sits there in bliss, uncaring as blood begins to dribble from the long, long cuts down his sides and from his neck where Derek keeps puncturing it. He quite likes it, actually, as he stares in the mirror. It contrasts nicely with his pale skin.
Derek just keeps cumming inside of him, shooting load after load after load it's almost ridiculous, and as Stiles doesn't share the same prolonged orgasmic high as Derek—clit softening—his mind begins to wander and wonder just where in the world all that semen could possibly be stored. Derek does have big balls, but they certainly aren't that fucking big… would have to be the size of fucking beach balls to keep all that… maybe werewolf cum is denser?
Finally, Derek mostly recovers from ejaculating, now just occasionally leaking a bit more sperm into Stiles' already perfectly full intestines, fucking overachiever. He looks in the mirror with lidded eyes which snap open as they see the red running down the curves of Stiles' body. "Aw, fuck, Stiles," he says, "I'm fucking sorry, I didn't mean t–"
Stiles waves him away, sitting on his knot. "Ay, whatever. They're tiny, they'll heal fast. And if not, they'll make some cool scars, all symmetrical and shit!" he giggles.
Derek's hands run up to wipe away his tears and drool, making him look a bit more presentable in the mirror. "Hey, I didn't mean anything I said. Like about you being a slut or having a small cock or whatever."
"I know, dummy," Stiles huffs.
"You did really good. That was fucking awesome."
Stiles smiles turning to look at his face without the mirror. "I definitely agree. We should totally do that again. As soon as possible."
Derek chuckles. "Actually, I take the whole not meaning it thing back. You definitely are a slut."
"Hey, well, you know how at the beginning I said I didn't know what I was doing leaving my window open and whatever?"
Derek nods. "Yeah. I wasn't really sure if you meant it or not, I just kept going so I could fuck you, honestly."
Stiles giggles. "Well, that's good, because I actually totally did know what I was doing."
The werewolf just shakes his head, knot twitching inside of Stiles' cunt as he half-assedly slaps his rear. "Fucking slut."