Well let me BIG TIME preface myself here by saying that I strongly disapprove of rape, that said, I can see how it would be an evolutionarily beneficial strategy for the male involved. See, for women it is, from an evolutionary perspective, strongly beneficial to select the optimal mate, one bearing good genes and who will stay to help raise the child, because for women the time and energy investment in having a child is more than a little bigger than it is for men. For men, however, while playing into that (IE, becoming the perfect mate) can be an adaptive strategy, another very adaptive strategy can simply be to mate as many times as possible. A male who commits rape doesn't have to stay around to raise the child, in the mere act of forcing impregnation he has achieved his evolutionary goal, and is free to move on and impregnate another woman. While these children have a lower rate of survival, because they won't have the same level of care, in the mere act of increasing the number of times he mates (and as a result, the number of children he has) the male has found a strategy which will pass the gene which lead to the mental state which predisposed him to rape.
Now that's not to say that there are not other reasons. There most definitely are societal and psychological issues that play into rape as we see it today.
Whatever the reasons, it's an ugly phenomenon. It's unfortunate that it is a part of our evolutionary history as a species, but unfortunately nothing has ever been prevented merely because it was unfortunate.