Well, that is a truth with exceptions.
Fanfiction.net does accept mature stuff. But they do not accept NC-17 stuff.
They used to do that years back, but as the site grew, they wanted a more clean immage, and banned NC-17 stuff. It seems more and more high profiled sites, are dropping NC-17 stuff.
Allthough understandable, lets face it most of these stories are crap, it is a bit sad. Because amongst all those crappy stories there once in a while can be found a real gem of a story. It is possible to write erotic stories with an actual inteligent plot. But nowadays they seem harder than ever to find, because this kind of stories are banned from more and more sites that wants to be viewed as more professional an decent.
FicWad is one of the last that caters to such stories, i think. I hope the site wil continue on.