Lord knows it's used in movies to be "funny" when it's really not.

I think another reason is that because rape to a lot of people is defined as penetration, having a man be forced into penetrating a woman to some seems impossible. Or in other cases people don't think that a man in a terrifying situation where he doesn't want to have sex can't get an erection, when in fact they can, and people take that as a he wanted it sign.