I think the fact that it's not realistic - the "animals" are anthropomorphic characters - is why they can get away with it. Bestiality is mainly a taboo because animals can't consent (and cross-species sex is be largely impractical in the real world anyway). But this takes place in a fantasy world where animals can think, speak, and consent (and ostensibly have different physical biology to boot). Sex takes place in science fiction and fantasy worlds all the time - whether or not it's actually shown on screen or discussed.
Well, taboo is a cultural thing. What is taboo in one culture (or subculture) may actually be encouraged in another. And many times, people don't even know or think about the reason WHY something is taboo, they simply choose to accept that "it's gross" or "it's wrong" and do not think for themselves.
Of course there are other possible reasons why it is taboo in a particular culture, such as it being taboo in Christianity because the Bible says whatever it says about it... but I was speaking in what I see as the most broad terms, without reference to any specific ideology, and with my own considered opinion.