Why do girls think its funny to hit someone in the nuts then freak out when someone punches them in the ?

I've noticed that a large amount of girls seem to think its not quite but almost funny to kick guys in the nuts but at the same time think that its just plain evil to hit a girl at all, and im just wondering why because as far as i can tell teenage girls are just plain out retarded with matters of fairness and rationality (duh). so anyone got some reasons for this?