While I personally definitely am not okay with human/animal bestiality, Dark Avenger brings up a interesting perspective. I'm going to pounce on this and go and bring up a few things.
Bestiality is, technically, "natural", and occurs quite a bit in the wild.
Have you ever heard of a "liger"? It's a cross between a lion and a tiger. Well, these are usually called "hybrids". Other hybrids exist as well. The beautiful Harlequin and Catalina Macaws are both hybrids, and are technically the result of "bestiality", if bestiality is defined as two living beings of different species procreating. In these cases, they produce nonviable young. In the cases of the liger and these special macaws, they are bred by humans.
But in nature, hybridization occurs naturally as well. In North America, two species of birds, the Gold Winged Warbler and the Blue Winged Warbler, often hybridize, often to the downfall of the former species.
These two breed and produce several types of hybrid warblers, such as Lawrence's Warbler or Brewster's warbler. These birds are nonviable.
Now you might ask - what makes human bestiality so different?
There is quite a big difference actually, and it goes down to taxonomy.
Humans only have distant relatives.
Humans are the sole living member of the genus
Homo. This may not seem important, but it holds great genealogical importance - Genus is the next step up from species. While usually species between a genus can interbreed, the more distant the genuses, the more incapable of successful breeding there is. The warblers above are of the same genus. Same with the lions/tigers, and the macaws. Humans would not be mating with anything in their genus except for other humans, as all other
Homo species are long gone. In fact, the common ancestor with our closest relative, the Chimpanzee, existed
6 million years ago. The kind of bestiality that humans have are going to be completely unlike that of hybridization, which is the closest relative to such in the animal kingdom.
Note: It should be noted that there are cases of interspecies sexuality but this is usually considered a sort of way to "relieve" the "attacking" species and is technically more like assault than actual romance.
Does an animal have "consent"?
Legally, no.
But this brings up an interesting question. Consent is derived from conscious ability to make a decision about something, specifically in this case whether or not to engage in sexual intercourse. Animals DO show "consent" towards each other in a species. For example, female dogs that do not want to be mounted will often attack the male and force them into a submissive position.
However, it is important that we do not
anthropomorphize as humans animal nature. And this, personally, is where I think the whole "bestiality is wrong" thing comes from. It isn't necessarily that animals can't say "no", they most certainly can - it's that humans and animals do not experience the world in the same way, and thus when an animal appears to say "yes", it may not be the same kind of "yes" that a human understands.
Some animals, like birds, have a very short period of sexual intercourse. It only lasts a few seconds - the male pecks the female's cloaca, and mounts the female quickly and the whole thing lasts only a few seconds - outside of the difficulty of balancing, that is. It is unlikely that birds share the same sexual experience as human do. Thus, even though a bird may not "peck you back" if you were to engage in sexual activities with it, it may still not warrant such a reaction.
Another thing to consider is that both species will consider different actions "sexual". For example, parrots consider rubbing the back and belly to be sexual actions and will arouse the bird if you do this (because it can trigger hormonal aggression, this is not recommended, even if you are a disgusting pervert!).
But what about fetishes?
It's true. Most of us all have a dark side. For some, it's vanilla, others, a lot of leather and cracking whips, and for even others still it's unusual things such as inflatable pool toys or eating chocolate. Everyone is different. However, at the end of the day, the question that makes these things "okay" or not is, at its core, "Are they both communicating the same message of consent to one another?"
In the case of weird, out there fetishes, this is usually not a problem since it is an inanimate object with absolutely no capacity for sensation, feeling or thought. In other cases, it depends on the person on the other end - did they say it was okay? Do they have the capacity to say it's okay? Young children for example have no idea what certain things may mean or imply and using them as such is exploitation of their innocence - while they may discover earlier than others, a set age is placed in society so that a reasonable line can be drawn.
In the case of animals, again, we have to take into consideration the fact that animals perceive their worlds completely differently than humans do. To them, their "love" may be very different than what you feel as "love". To claim that they "love you" (by default in the same sense that you perceive love) is questionable, because you cannot empathize with their potentially different definition of love, and vice versa. They do not understand not because they are feeble minded, but because their minds are different and they experience the world quite differently than you or I.
Final Notes
It should be noted that the concept of "consent" is quite a human concept. Many species engage in outright rape (such as mallards) with no social intervention or punishment. To a mallard, this is a completely acceptable practice, and it's likely acceptable to perform such a deed on other species as well. Adelie Penguins not only commit rape but have been recorded performing necrophilia. Many species of insects kill and eat their mate afterwards. Nature is not pretty and will not conform to human "decency". It is likely that bestiality's taboo nature evolved due to the health risks associated with it.
There is a reason why horse-****ing is probably not in your best interests.
Morality is truly a human concept, and thus nothing is truly "always wrong".