I think it's because atleast with genocide it's often for a point, be it domination, well intentions gone extreme, or for the Joker, if there's a good laugh in it. Rape, atleast I think, would be looked down even by the Joker (who does have a quasi-sexual side but really doesn't find things like rape funny, atleast that's what I get from his appearances) and it certainly doesn't achieve anything like a good ol' genocide does for Ra's, Grodd or anyone else.