First of all, one has to differentiate things: the law doesn't exist to promote particular morals per se. No matter how morally repugnant some act is to me - like, say, zoophilia - the law has no business imposing or protecting my views as long as that act doesn't affect my rights or those of another person. So, not everything that is immoral should be illegal - and what is immoral is frequently highly subjective. The liberty to make moral choices is, or ought to be, sacred and an essential part of human freedom.
So, the fact that you find zoophilia repugnant or sick is not an argument. We can't just start to make illegal what x or z consider "sick" and " ed up".
The consent argument isn't appropriate, I think.
First, it's obvious animals can't give their consent by speaking or writing, in a conventional way. However, one could argue that they are able to offer tacit consent on their own way, I suppose. For example, should we illegalize the use of artificial insemniation in the breeding of cattle, dogs, etc.? Many times the methods to collect semen are somewhat arcaic; the only difference is that the breeder doesn't take any kind of sexual satisfaction from the act. Do the animal give consent here? I think it's perfectly plausible that this kind of act can be neutral or even pleasurable to animals.
Second, there's a clearly double-standard. Do you go to the circus? Aren't the minds of circus animals "conditioned" as well? Aren't they trained by individuals who abuse "their simple minds"? And what about, say, castration? People castrate their pets all the time. Or perform scientific experiences. Do animals give consent in those cases? Is it okay to castrate an animal without consent but people should be arrested for sexually pleasuring them?
Third, consent it's simply a concept not applicable to animals. Do any of you own a pet? Do you keep it in your house? If so, has it consented? If not, should you be prosecuted for kidnapping? What about the hunters, should they be charged with 3rd degree murder? I mean, it's okay to kill an animal for our convenience or mere pleasure, but those that merelly jack them off should be arrested and tortured?
Again, I can think of plenty of arguments contra zoophilia. One can defend it's morally repugnant and a violation of our moral duty towards animals. However, as long as there is no pain or harm to the animal and the beast is not property of another person, there's no reason to make it an illegal activity.
Finanlly, it's truly amazing that people discredit the ssexualism/zoophilia slippery slope and simultaneously equate animals to children. Say it slowly: children are not animals. Quite disturbing, to say the least. Be assured that I don't defend you should be arrested though.