Chan is one of those grey areas for me. It mostly depends on how young the character is. I know how I felt sexually as a late teen and what I did. My absolute lowest age for penetrative sex is 16. I can do 14 for mutual masturbation and oral, but only if I really like the pairing and if the other participant is also a teenager, because that's seriously skirting the line for me and anything younger I just can't do. I quit reading Piers Anthony's novels for that reason.

Mpreg squicks me the HELL OUT. It just really, really does. I can hardly handle babies coming out of women. :P

My thing is sentient nonhuman sex. Sex with aliens or human hybrids or highly evolved AIs. Well, I reckon the last one might be possible in RL someday, but anyway. Technically, sex with aliens happens whenever someone has sex with Teyla or Ronon, but I'm talking about nonhuman physiognomy. I can eat Bug!John fics for breakfast, and that one fic where John was a centaur? *rowr* This is not to be confused with bestiality, since animals are not sentient and can't give consent, and therefore goes on my squick list. And then there's wingfic, which I always thought sounded really stupid until I got into SGA, and now the thought of John having wings is just really, really hot.