...but I can't really think of an example of a funny rape joke I've ever heard/seen, either.
We've all heard a tonne of male prison rape jokes. I rarely see the humour in them. Over used.
As for female rape jokes, the only one I can think of is Brian's mother in "The Life of Brian".
EDIT: Almost forgot there was one in an early season of M*A*S*H that made me cringe.