I think the question 'why do men rape' is quite a lot like the question 'why do black people commit murder'?

which is to say that the majority of men don't rape (and the majority of black people don't commit murder)...
people other than men rape - women for example (and people other than black people committ murder)...
and people do the things they do for a variety of reasons (though there may be some interesting generalisations to be made, i'm not sure that the categories are optimal categories for scientific investigation).