Katsuki reads and rereads that a couple of times because it feels very out of left field. What?? How did they get from kissing to sleeping?

Depends, probably.
On you and on the people you're around.
Definitely don't trust sleeping around certain people because they're dumbasses.

Kaminari is definitely a prank-people-who-are-sleeping type for instance. Sero too, honestly. So. Yeah he doesn't trust them.