Stiles is pretty sure that if Derek moves away, he's going to die. But he's also pretty sure if Derek keeps going he's going to die. And the kicker, his brain helpfully reminds him as he lies there, sucking in deep breaths and trying desperately not to come, is they haven't even really gotten to the sex part yet.
He's horrible at this. He's awful. It's like he went from high school to graduate school, and he's failing so horribly. He should have gotten more experience before thinking he could have sex with Derek Hale.