I think that on a level the question of why do men rape may be one worth analysing, on some basis of a better understanding and on a very thin basis of prevention, but i don't think the answer to such a question is worthy in any basis of healing, the why questions never are (unless it is to break down false ideas). There is no justification in any response to why. But then i guess this is meant more as an intellectual debate.
Intellectually i don't think there is much of a discussion needed, like Lance said, it is about sick power. And quite rightly, like Lance said, it is because they are monsters. I don't know which comes first, but there is no denying in any way that somebody who rapes somebody else is a monster. Any book or any theory that tries to answer the question of why men rape that fails to boldly and most prominently call these people monsters and instead tries to make excuses or clauses babbling about evolution is quite frankly not worth my time. I understand the trap that analysts fall into when they have topics like this to study, they need to have enough to talk about to publish a book or put their name on a research paper, but really it is a simple thing and Lance has written the arguement out in full in those few lines.