Consent
There seem to be a lot of arguments on this board revolving around the issue of consent. Some say a horse (or any non-human animal) cannot consent to sex with a human being because it lacks the cognitive ability. Others argue that the fact that some animals initiate such sexual activity shows that they can offer meaningful consent.
I can see both sides to the argument, but I think this debate misses the point entirely. Animal consent is an irrelevant topic in the context of our current legal structures. Completely irrelevant. There is no other area where an animal's consent is a valid consideration. Under law, we can do whatever we like to them—lock them in a cage, make them pull carts, shackle them, brand them, castrate them, euthanise them or slaughter them for food; the one condition is that we don't cause them unnecessary, gratuitous harm. As far as the law is concerned, animal cruelty laws are as far as animal welfare concerns go.
Perhaps you're a radical vegan who thinks no animal coercion of any kind should be legal. That's fine, but our society does not come close to sharing your code of ethics. You might want to try ending factory farming first before worrying about the niceties of animal consent.
So, this whole issue of consent is a red herring. What people should be asking is the following: is bestiality a form of animal cruelty? Are there cases where it isn't? And if—in the case of the latter—there are, then should they really be considered unlawful or unethical?
Just quietly, I don't actually believe that the bulk of the disgust aimed at bestiality comes from genuine ethical concern for the wellbeing of animals. I think the issue of consent, when it arises in this context, is mostly just a post-hoc justification for an irrational opposition to 'perverse', 'unnatural' sexual behaviour—a category that once upon a time included homosexuality and miscegenation before racism and homophobia became unfashionable. That sort of bigotry may be understandable as an uneducated knee-jerk reaction, but it should not be allowed to dictate law.
None of this is to say that I condone bestiality. To be honest, I find it a strange practice, and wonder why anyone would want to participate in it. But, so long as no serious physical or psychological damage is being done to the animal, I do not think it should be illegal. It should not be in the state's interests to send people to jail for no good reason.