Oh good lord. That's the sort of thing we moved to IJ to avoid! If fic is illegal then we must take it down - if it is not, then we shouldn't need to sneak about with it. As I understand it, it's in a legal grey area, and the policy is that if it is complained about, it is taken down. As it is, I think I'll keep the MPAA rating system, since it's what's used in fanfic generally, and if we get complaints and get put down then so be it - it won't be because I did something different than all the other fic communities.
I think I will put a disclaimer in the community info, though...