I had usually held the opinion that bestiality is wrong, not because the idea of sex with another species being somehow "unnatural" or "immoral", but because animals can't give consent, and therefore bestiality is by default rape. However, after reading How to have sex with a dolphin, I have realized that that is completely untrue: if an animal approaches you, or is in some other way allowed to choose to initiate the sexual encounter or not, that could be perceived as consent, no?

Yes, sex with animals is icky. Yes, it's a good way to encourage interspecies transmission of diseases, especially when with primates. Yes, it's socially unacceptable. Yes, it's bizarre.

But is it wrong?

My general opinion has always been that all sex that other people have, as long as it is consensual, is something that's OK, whether it appeals to me or not. Given this principle, it appears that whenever an animal consents—as many actually are able to do—to have sex with a human, is completely acceptable.