"I'll change back by morning"
Okay... I... I have to know. Say that they are, erm, having carnal relations at from night until morning. And, when that time comes, he's still inside... What happens? Just it just pop out/get pushed back out?
(And, no, it wouldn't have anything to do with the ass. Unless he was already in there to begin with... Which I guess wouldn't be a problem? ...I guess...?)