Hehe. Not forbidden, but what is usually warned for is non-AU RPF (as in J2, as they are, in real life, with families and making Supernatural), infidelity kink, and there is usually a large amount of AUs involving all of those things applicable to SPN slash that you mentioned + things that are generally triggery for some people like priest AUs, age-difference with one player most usually underage, and religious kinks. But it's kinda-sorta the thing that you associate with Lolita being "erotic": it isn't going to be okay at ALL in REAL LIFE, and nobody's saying it is, but in fic it flies, and under good writing it flourishes. (And there are a lot of good writers doing RPF in LJ.)

And I don't know how, but even in AU RPF there are quite often commenters saying the characters are "in character", so there is something I'm missing there.