I agree with shinigamiinochi. If you want to write or read something, you should have the right, regardless of the content. I believe that everyone should be welcome to do what they want, so long as it doesn't hurt anyone else. Writing fanfiction, even that which involves underage people, isn't hurting anyone. No one is forcing anyone else to read it.
Free speach is a very important thing, and restricting content is taking that away. I love the fact that this site isn't restricting anyone. I also have a fic that involves rape, even though I also believe that rape is disgusting and should be a heavily punished crime. There is a big difference between reading and/or writting and the actual act.