Because we follow our insticts, but some of them are more acceptable than others. I think as society evolves, the concept of sex as 'nasty' is slowly dying out, but unfortunately, the concept is being replaced by the notion of sex as a fashion accessory, which is different, but not necessarily any more desirable! As far as violence goes, I believe that our veneer of civilisation is just that - a veneer, and a thin one at that. Just below the surface is our violent inside, waiting to be tickled and prodded - not nice, but the way we are, until a few million more years of evolution straigten us out as a species. It takes a lot of time, so don't hold your breath!