Derek would bare his teeth in anger at first and then he’d start shivering all over, lip a sweet tremble that he sucks into his mouth as his orgasm hits him. His eyes would go all lost with longing, and he’d stare at Stiles in desperation, knot pulsing, come rushing all white down his hairy legs.
Stiles would just freeze there in the door, mouth gaped.
And for months after Derek would be this surly, frustrated beast about it because when you knot in front of someone, there’s this bond and he’s been saving that for his girlfriend. They’ve been having all this heavy petting in the backseat of his car, his hand up her skirt, his fingers tracing the delicate lace of her underwear, their mouths all panting together. He’s been DYING to give her his knot for ages and he’s been so patient and waiting til they’re both ready, courting and kissing and meeting PARENTS and he PINNED her—?
—and now he’s got this super-intense knotting bond with this loud-mouthed, dorky freshman kid and—
—he just wants to put his teeth in Stilinski’s throat and drag him out to the woods and mate him. Werewolf sex isn’t even mating, but what Derek wants to do to Stiles is mating. Mating in the dirt with his wolf hips flexing all rigid and frantic. He wants to mate that fucking brat up, wants to blow a few knots in his soft little hole, wants the kid to whine helplessly like a hurt animal under him.
He’d go out with his girlfriend and hold her breast in his hand all warm and welcoming at makeout point, their tongues working together, but after he’d drive over to Stiles’ and the kid would get in his car all breathless and excited and Derek would drive back to makeout point so he can get between the kid’s willowly legs in his backseat and grunt-rub his knot off on the kid’s fat crotch.
When he comes, he sometimes howls, all wolfed out, shakes through it to the confused sounds of his pack howling back.
One night, he growls all low and threatening through Stiles slowly, carefully unzipping his tight fly, opening his pants. He’s got furry, clawed hands by the time Stiles gently, tenderly eases his knot free from its confines. And then Derek’s growls turn into little whimpers as Stiles ducks down and strokes his soft little tongue over his knot, wetting it, marking it.
He just drops his head back onto the seat, eyes staring sightlessly at the ceiling as Stiles plumps his lips around part of his knot, sucking, trying to play with him, trying to get him off.
When he comes, Stiles moans and goes a little crazy, holding his pulsing knot still so he can suck it hard and passionately, cheeks a red, spit-wet mess, eyes all closed with bliss.
After, Derek’s like, double-knot bonded to him after and starts following him around like a puppy.