I think the only way they can be 'funny' is if you are someone who has only the vaguest sense of what rape (disclaimer: here, obviously, I mean rape as it is presented in the myths) is.
First you have to understand that women are mostly 'up for it' anyway, and the idea of not wanting sex is a joke. Then you have to assume that rape only happens rarely, to women who really want it anyway, and who by the way are probably poor, silly fools who did something wrong like walking up a dark alley.
Then, like quite a lot of teenage boys, you can find it really funny. That joke about it not being rape if you shout surprise was doing the rounds when we were teenagers. The issue isn't whether or not these people genuinely laugh - they do - it's that in order to laugh, you have to have already accepted a lot of rape myths and myths about women, or you have to have turned off what should be a natural social revulsion.
You watch an audience and some jokes just don't work - people go quiet or boo. IMO that is a good example of when a social revulsion comes in and protects us from trampling all over something serious. I don't like the idea this should be a challenge and I think with rape jokes it has become one, so some comedians have been pushing and pushing to see if they can desensitize people a bit more and make it funny.