Oh, my squickability is quite high. And I agree, incest/kiddie porn is WAY high on the list. I saw a beautifully drawn comic over on Rule34 not too long ago that literally made me nauseous; it was 10-year old Bruce Wayne being "comforted" by his mother. *Shivers at the memory*
But I've been in the fic part of fandom for going on 15 years and seen just about every pairing imaginable. I'm heavy into Star Wars fandom, and I STILL see a Luke/Leia pairing now and again. I guess my point is that if the characters have genetalia, somebody's going to write about them having sex. I go to a con every year that basically a fic con, (they actually still put out paper zines) and run through the new stuff and go "no, no, ick, ew, what were they thinking, no..." (somehow I still wind up buying like $200 worth of zines every year though...)