Actually, I think they are afraid that the pretty girl they want to rape may just be armed and willing to stop the rape by shooting them. And that isn't fair!
I remember a class called Philosphy I had in college. We had a student in class argue that it was wrong for a woman to resist a rape since she would be preventing the male from doing something that was "good for him". Notice that he said nothing about her doing something that was good for her!