I personally and i can not talk for other fanfic writers and readers or for that matters any writter in general, but i started fanfic writting and reading as an escape into fantasy from my own at the time harsh reality.
Though my reality hasnt improved much since then, I enjoy the escapism that these story give me. As a writter i enjoy occasional pushing the bounderies between what is taboo in this world and the world i write in if that makes sense.
I have written rape, incest, child abuse and many other things in the past,, but i do not condone these acts in anyway. As for safe sex well it kinda kills the flow to have one character stop to put on a condom or diaphram.
Stories dont have to be reality and thats what makes them such a great escape from said reality. The problem lies i think when people take these sometimes violent and otherwise controversial storylines and plot to heart. We write fun dont we?
Any oen who takes these themes too strongly should not be reading or writting this stuff to start with. Writting and reality are to things within the world of fiction that jsut dont mesh, fiction is our own escape into fantasy.
People use to love reading books like treasure island and sherlock holmes, but did those people grow up to become pirates or vigilante detectives? I think not.