Most of them genuinely believe it, too. Everyone knows rape is evil, and everyone thinks that they're a good person, and since good people don't do evil things, they can't be rapists. They'll do all kinds if mental gymnastics to avoid critically examining whether any of the sex they've had was actually rape. She was asking for it, he couldn't help himself, she didn't say no, she was into it at first, she was leading him on, she just wants attention. These guys will come up with any excuse that keeps them the "good guys."