Why was the nun mad at the altar boys? They were a bunch of cocksuckers.
Is this some kind of religious sexual joke?
You nailed it! The nuns are mad that the boys are too busy sucking off the priest instead of sleeping with them!
Joking about children being molested is pretty messed up. Child molestation is one of the worst things that could ever happen to a young child, so making light of these situations is a little insensitive. It's also rude to generalize religious figures as child molesters.