Grendel, by your reasoning, dogfighting and cockfighting should be permitted, right?
Why are such activities illegal?
Our ideas of animal rights are anything but consistent or based upon some universally accepted ethics.
Rather they are based upon arbitrary cultural practices.
Thus a human killing an animal for sport (i.e. hunting and fishing) is accepted and legal, though not always quick and painless. While animals killing each other for sport is not, even if it was quick and painless.
Moreover, it is considered immoral and illegal to kill animals capriciously--provided that the animal is not an insect. The cuter and more endearing to us that the animal is, the more we take offense and find it immoral.
(I have facetiously argued with my son that if evolution is correct (I do believe it is) that as more humans populate the earth insects will evolve in cuteness and appeal in order to survive better. Kill as many ants as you want, but oh don't harm the cute little lady bug.)
So, for the sake of argument, let's put aside the consuming animals without their consent.
Is it alright to use animals physically for work or for entertainment, recreation and amusement without their consent?
Before you get on the back of a horse and ride it for miles do you ask permission from the horse?
Of the two ways, there is only one way of riding a horse that is not perverse; yet both require a human being taking advantage of the animal without consent.
The line is drawn at sexual use, but is that line arbitary?
I, of course, believe bestiality is perverse because it is contrary to our human design--our physical form and function dictate the fundamental nature of our accepted behaviors and as such our ethics based upon a natural law. We are designed to eat and digest meat; therefore, the eating of meat is ethical. We are designed to have sex with one another, thus sexual relations between humans is accepted and inter-species sex is not.
Is regard to sex between humans, this is where it gets sticky (pardon the pun). Does the design of nature point us to having sex with members of the opposite sex? I think the answer is an unequivocal yes. Does the design of nature point us to having sex with members of the same sex. The answer is a unequivocal no. Tab A was designed for Slot B.
But to hell with that, right? Why should we deny a person's inborn desires to any other human based upon some kind of natural design? Anyone who finds it disgusting is simply being narrow. Fine, I don't have a horse in this race, but I do ask how can we thus sit on (and ride) our high horse and tell those who find physical pleasure with animals that it is contrary to nature?
(By the way, when Fido willfully and happily begins humping your leg, I think the consent is implicit.)