Same age sterek, established relationship, neither knows the other is supernatural yet
One time meanwhile they were having sex, Derek bit down on Stiles shoulder and neither one of them thought much of it in the moment, since the moment was both of them on a high from the sex (and the bite, but they don't know it yet), then when the euphoria has cleared a bit, Derek has a brief panic that Stiles might find him weird for biting him (especially since he did it hard), but Stiles kinda just says aloud, "huh, I guess I have a thing for biting"
It's not until much later when he's back home does Derek realize that he gave Stiles the mating bite and begins freaking out again
Meanwhile, right after Derek left and Stiles took a shower, he put a glamour on the bite so no one, especially his dad, could notice it
So the next time Derek catches a glimpse of Stiles' shoulder, whether in the locker room or they're having sex again, he's shocked and relieved to see that there's no trace of a bite, so he doesn't have to worry about telling any of his family what he did or having to reveal to stiles that he's a werewolf and what a mating bite means, but then why does he feel kinda disappointed that Stiles isn't wearing his bite? There's not much time to dwell on it cause the next time they're having sex, Derek is biting down on that same spot again (disspelling the glamour Stiles had put on it) and there's the bite again, but once again Stiles is hiding it
This cycle continues and grows to the point Derek's biting him when they're just making out, and it only ends when Stiles gets the urge to bite Derek back and he's just unintentionally sealed their mating bond, Derek's once again freaking out and debating whether or not to finally tell his family, at least Peter, and Stiles is finally noticing a difference in his magic, and it all comes to a head when Stiles sees his bite on Derek and realizes it's been long enough that it shouldn't be there anymore, and because of the mate bond, Derek's finally able to smell the magic on and around Stiles (maybe even see thru the glamour and see the mating bite scarred and deep from the amount of times Derek's bitten him)