Stiles doesn't know how it happens, but it does.
There's a school girl costume that he doesn't want to know where Peter got it from, thigh high socks, skirt and panties, and he feels like he should be worried by how much it turns him on, by how much he wants this, but he doesn't, not when Derek's staring at him like this, like he wants to fuck Stiles right there, not when he can see Peter's smug smirk, cock pressing against his trousers.
He's laying on the bed, chest flushed and skin pebbled, and Derek's near his head, looking down at him with dark eyes and wet lips, he's got his trousers open already, cock in his hand as he stares at Stiles, and Stiles can't help but lick his lips, wants to taste Derek on his tongue, feel the heavy, warm weight of his cock in his mouth.
"Derek." He croaks, and there's movement from across room, but Stiles doesn't pay any attention, not when Derek shifts, his knee coming to rest on the mattress, and Stiles opens his mouth, tongue flicking out again, eyes pleading up at him, and he's so gone already, cock leaking against the panties, and he can feel the plug that Peter put in him earlier, rubbing against his prostate when he moves, can't stop the moan that bubbles in his throat.
He can see the moment Derek gives in, watches as he pushes his trousers further down his thighs, before he rubs the head of his cock against Stiles' mouth.
Stiles licks across the head, tongue dipping into the slit, and opening his mouth more when Derek rocks forward, cock hot and heavy.
He swallows, throat working around Derek's cock, tonguing at the vein on the underside, spit slicking his lips and dribbling down his chin, Derek's moaning above him, hips stuttering like he can't hold himself back-
and Stiles realises very quickly that he doesn't want Derek to hold back, tries to relax his throat and curls his fingers around Derek's hips, pulling until Derek finally gets it and starts thrusting, fingers curling in the back of his head.
"Fuck, Stiles." Derek whispers, voice wrecked already, and Stiles moans, mouth sloppy, and throat working, hollowing his cheeks with each of Derek's thrusts.
There's the sound of rustling fabric, the clatter of a belt falling, before there's another dip on the bed, warm hands curling around his ankles and dragging up to his knees over the fabric of the socks.
Stiles doesn't look, too busing swallowing around Derek's cock, lashes wet against his cheeks, but he can imagine the look on Peter's face, dark eyes and curled mouth, can only imagine how he himself looks, spread open, mouth stretched wide and whining, he can tell there's a wet spot leaking onto the skirt, stomach sticky with pre-come from where his cock's trapped under the fabric of the silk panties, and he wants them off as much as he wants them on.
"Look at you," Peter says, fingers dipping under his skirt, blunt nails scratching across the skin. "So eager, sucking his cock like a good boy."
Stiles fucking whines, cheeks flushing more, and hips rocking closer to Peter, tries to get Peter to do something, anything.
"Mm, you look so pretty like this, so wet." Peter hums, one hand curling around Stiles' hip, the other lifting the skirt, palm catching across his cock, and Stiles chokes, swallows around Derek's cock, throat working, and tears in his eyes. He can feel Derek falter, fingers soothing on the buzz of his hair, other hand rubbing at the corner of his mouth, and when he looks up, his eyes are worried, but Stiles manages to shake his head, pulls his head back so only the tip of his cock's still in his mouth and sucks, tongue flicking across the slit.
"I think I should fuck you like this. You're already stretched open aren't you? So needy, I bet I could just slide right in and you'd cling to me."
Peter's closer now, Stiles can feel the warmth of his body between his thighs, the brush of bare skin against skin, and wants him closer, wants to be filled and fucked, taken apart piece by piece, wants Derek to fuck his mouth when he comes.
Stiles pulls off of Derek's cock, tries to ignore the cut off moan Derek lets slip, curls his fingers around the base and jacks slowly, head turning towards Peter.
"I want you to fuck me." He says, and he's almost surprised by how raw he sounds, throat scratchy, and lips feeling thick. "I want- I want Derek, I want you to come in my mouth."
Derek groans, head falling back, but Peter's smirk only curls a little more, eyes sharp.
"On your knees."
Stiles scrambles, limbs heavy, sock covered feet slipping on the bed, but he finally manages to get onto his knees, Derek's cock in front of his face, tip wet, and Stiles wants it back in his mouth, wants to make Derek fall apart, wants to watch him fall apart, but before he can lean forward, Derek leans down, tongue licking at the corner of his mouth before he kisses Stiles, mouth soft and almost careful and Stiles closes his eyes, falls into, nipping at Derek's bottom lip, before they both pull back.
There's a slap and Stiles jolts forward, he can see Derek turning towards Peter, eyes narrowed, but Peter laughs, eyes probably flicking down to where Stiles had rocked back against his hand.
"That's all very sweet, but I think Stiles asked for something else, hmm?"
Derek doesn't stop staring at Peter, who huffs in return, but he does nod, fingers curling around Stiles neck.
There's stillness for a moment, and Stiles wriggles his hips, is sure he looks stupid, but then there's hands pulling his cheeks apart, can imagine the silk of the panties stretched over the plug, hole red.
"Fuck." He manages to croak at the sudden cool air on his skin.
It seems to spur Peter on though, who pulls the panties to the side, holding it against his left cheek, the other hand going to the plug and teasing, pulling it out slightly before pushing it back in, and Stiles whines, head falling down, before Derek pulls it up, guides him back to his cock, and Stiles almost drools, throat swallowing and eager, the tip almost sweet when it finally touches his tongue.
Peter plays with the plug a little longer, until Stiles gets impatient and tries to pull himself away from it, rocking his hips forward. Peter pulls the plug carefully and Stiles can feel the stretch of himself around the toy, whines around Derek's cock at the sudden empty feeling, hole clenching on nothing.
There's the sound of lube being opened, the slick sound of skin on skin, before the cold gel is rubbed around his hole, Peter's finger dipping in briefly before moving away.
Stiles shuffles, cock leaking, and sticky against his stomach, hole fluttering, skin flushed, body eager, trying to swallow Derek down.
The blunt head of Peter's cock against his hole almost shocks him, but Peter pushes forward, head pushing and catching against his rim, before he pushes completely in and Stiles groans, wrecked and spit slicked, eyes fluttering and stomach clenching.
Peter gives him a moment, before he pulls out, hips holding still, and Stiles starts to push back, needs Peter to fuck him, but Peter slams forward, Stiles rocking with the pressure, Derek's cock catching his throat.
Peter fucks him like that, thrusts strong and sure, catching on his prostate, and Stiles can tell his face is splotchy with tears, half bitten off sobs catching in his throat, hole raw and trying to clench down on Peters cock, muscles rippling.
He's only suckling on the tip of Derek's cock, tonguing at the tip, mouth slack and open, but Derek keeps on making broken sounds like it's the best thing he's ever had.
Peter leans forward, fingers tight as he grips Stiles' forearm, pulling it back until his back bends, neck taut, and uses it to rut into Stiles, pulling out only a little before jolting back in, the breath knocked out of Stiles each time, and Stiles feels so wrecked, taken apart and used and perfect, almost wishes he could stay like this forever.
Stiles manages to wrap his lips back around Derek, hollows his cheeks and sucks as he tongues just under the head, hums happily around it, the corners of his lips feeling raw, spit slicking his chin, and Derek tastes so good, sweet and salt and warm, heavy pressure and Stiles wants him to come, wants to feel him fall apart in his mouth.
He flicks his eyes up, stares up at Derek as Peter fucks into him, whole body flushed and thrumming, and it's a sweet kind of ache, his panties soaking through, cock so hard and desperate to come, but he wants them to come first, needs them to.
Derek's staring down at him, eyes flicking from his mouth, neck, back to his eyes, fingers burning into the skin of his neck, thumb digging into his collarbone, occasionally rubbing in small circles. Stiles hums, wishes he could use his other hand to curl around Derek, fold his fingers around his balls, settles for tonguing at the slit and hollowing his cheeks again, doesn't take his eyes off of him, even though they want to flutter close.
It doesn't take long, Derek starts to make small thrusts, hand moved to cupping the back of his head, gasps leaving his parted lips, and his whole body clenches, back bowing forward, hips rocking forward more and Stiles swallows greedily, throat working as he tries to get everything, some slipping past his lips anyway as Peter slams into him.
Peter's thrusts are getting sloppier, harder, his fingers digging into the skin where he holds Stiles' hip and arm and Stiles knows there will be bruises later.
"Fuck, Stiles you should see yourself, so greedy, swallowing everything he gives you, begging for more." Stiles whimpers, and Peter pushes down against his back, between his shoulders until he falls forward, face smooshed against the bed, cheek wet. "You're so tight still, clenching around me."
Stiles sobs, the taste of Derek still on his tongue and his lips, aching to come, only needs something else, but doesn't know what.
The bed dips in front of him, and he can see Derek's leg tense as he shifts over him, and Stiles wonders briefly through his muffled head what he's doing, and then there's extra pressure against his hole, Derek's finger pushing in along side Peter's cock and Stiles can't- body tensing and hole clenching as he comes, buries his face into the soggy fabric of the bedsheets, desperate wet whines leaving his raw throat, cock pulsing against his stomach, the panties even sticker now.
He dully feels Peter come over the rush of his own orgasm, the roar in his ears, the murmur of "Stiles, fuck, Stiles, so tight." feels the come dripping down his thighs and over his balls, and whines when Peter pulls out, before he collapses into the mattress.
Everything is muffled from then, knows that Peter leaves and that Derek cleans him up, mumbles sleepily as he's moved around, the sweat slick clothes pulled from him, his stomach and hole washed carefully with a wet cloth, manages to smile when Derek kisses on the forehead before he finally allows himself to sleep.