On page 90-91 of his Letter to a Christian Nation, Sam Harris tries to argue that religion possibly helped prehistoric humans to "socially cohere" in the past, but it no longer serves this purpose now. He then draws a parallel between religion and rape by writing: "There is, after all, nothing more natural than rape. But no one would argue that rape is good, or compatible with a civil society, because it may have had evolutionary advantages for our ancestors."
I am not sure what Harris mean when he says that rape is "natural." In the context of this utterance, it seems that he is trying to say that rape is part of our so-called evolutionary heritage or that it had certain advantages vis-a-vis our evolutionary development in prehistoric times. I am not sure what advantages rape supposedly had at one time nor is it easy to figure out what Harris fully means by "natural." Once I find out, I will edit this post to more adequately address his comments.