Man, I'm gonna get so many haters for this, but...

TMNT. April x Casey I'm okay with having a life like that (not that I care to see it), but the turtles themselves? No.

Also human x feral pokemon. Not okay.

Okami. Ammy x anything makes me sad. *Chibiterasu* x anything makes me rage.

There are more, but these stick out like sore thumbs for me. Mind, these are personal opinions and I can't well explain why these are bad and others I don't care for, but there you go.

Ugh, I have bad associations with TMNT and porn due to a bad relationship some time ago. I can't handle the concept as a result.

I think a lot of things should be "off limits" but there are other people with the unhealthy fascination of turning something pure into something raunchy... so it'll happen eventually. Some people actually have crushes on the characters from childhood shows, and that's their excuse... but mostly people do it because it either (1) hasn't been done yet, (2) they think it's funny how people react to their favorite shows being violated, or (3) they don't have any interest in that "particular" show and don't know why people care so much, but have their own "certain shows" they won't turn into rule 34 because it's too close to them.

Not sure if I'm as likely to put it down to maliciousness. I think there's a certain culture among fur artists that basically rewards people for porn, porn, porn and more porn, and there are only so many anthro (or stuff that can be made into anthro) sources to draw from. Thus, endless Sonic, Renamon and Pokemon porn, to say nothing of Krystal.