lariii's feed

Can someone explain to me why monsters r*pe people? I have seen it many times but I still can't get my head around it. They are completely different species and lack common sense, so why even bother? I get simply killing or eating humans because of "survival of the fittest" but how does r*pe relate to that? Also if they do for some reason r*pe humans why stop at girls (or humans for that matter)? Not that I want to see it go both ways but why would they make distinction between genders? It doesn't seem to be due to a need for reproducing so there is no obvious explanation. Overall, the story has some potential (ignoring the many potholes) but it repeats meaningless and boring cliches that don't lead anywhere or/and aren't consistent. (For example why r*pe every other girl but keep FL to eat? Or is it only some monsters that r*pe? Then why some do while some don't?)