I think zoophilia/bestiality is far from wrong; consent is clearer in other animals, because they don't feel the need to avoid the social pressure to have sex. Plus, they won't hesitate to scratch and kick and bite if you do something that don't like.
This so much.
The issue is there is NO WAY to prove it. While you could argue its obvious, said animals can be "trained" (Read, abused) to do things they do not wish to do, and if there IS abuse happening the proper protections are not in place, and indeed, cannot exist efficiently. This is without discussing the health concerns (Both mental and physical) for both members of such a relationship.
In addition sexual relations require equal power between both members except where power is given up willingly, in the case of animal/human relations, this is impossible. This opens up the possibility of the relationship becoming abusive, possibly even by accident, and in such an instance, there would be no way of verifying if this has or hadn't happened within a relationship.
It all comes down to consent, and an animal CANNOT give consent that would pass in a court of law. "They looked like they wanted it" is not acceptable, and probably also a quote from several convicted rapists, and before you say "Common sense" let me point you back to the first paragraph, how do you prove the dog was not abused to make it more willing for sex?
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Oh, and of course it's an animal your trying to **censor**... that is kinda gross
((Yes, I know that last argument is super weak, and not even a real argument, but it had to be said.))