dolphins also seem to be favored by some; there's a website about it that probably gets most of its visitors in the same way as
goatse.cx.
But zoophilia is not just funny or gross. It's a perennial flamewar on de.alt.liebesakt, and some people there have argued pretty coherently that why the heck should people not have sex with an animal if they really love it and it's anatomically possible and not painful to either participant? The standard reply (except for religious arguments) is that just like children, animals can't say "no" because they depend on you, and can't even tell you "yes" at all. The standard reply to that that a horse, for example, is quite capable of and ready to communicating its disagreement, especially to someone standing behind it. The zoophiliacs also claim that the animals in fact initiates the sex in some cases.