I think (and I am just guessing here) the assumption is if neither sex is turned on by the idea of being forced to have sex against their will, a man will have an advantage when forcing a woman (because they can get hard if they are turned on by the idea of forcing a woman to have sex against her will, and thus be able to insert the penis in the vagina) where as a woman will find it more difficult to get the man hard (in order for her to have him stick his penis into her).
That is, of course, assuming she is using HIS penis to penetrate HER. If there are other implements involved (which I will leave to your imagination) then none of what I just wrote really applies all that much.
Plus there is the potential problem that if you play with a guy's dick for long enough it will get hard whether he wants it to or not. But that is another matter, and not something I think we should really get into discussing at this point, especially on a forum that is supposed to be PG-13 because I am certain I am coming close to crossing that already.
(And please remember - everything above is literally all guess work since I have not made a study of how to rape men or women. Something you are all probably glad to know).





