No different to when a man rapes a woman, in my book. Both are equally as disturbing and I don't think it's a good idea to distinguish between the pain that goes through both sexes in such attacks, as if one is any less worthy of our empathy than the other. Saying that, the sex of the viewer probably has some say on the impact on the suffering for the viewer during such differing rape scenes, even if it shouldn't really do so.