The same user you responded to also said this, and I was wondering how you'd respond:
"Exactly. And as long as the rapist was careful not to physically harm the animal too much, there would be no way to prove animal abuse. The presence of semen in an animal wouldn't constitute a crime.
Adam's argument relies on his feelings more than he would like to admit. The legal system needs some way to determine what is and is not animal abuse, and what is and is not legal consent for sexual activity. If sexual activity with animals were legal, animal rape would be de facto legal unless the animal became so scarred and physically/psychologically damaged that a court could determine beyond a reasonable doubt that the sex had been nonconsensual. Which would be near impossible. Furthermore, I don't believe animals even have a legal ability to testify or give evidence in court.
Our courts have a hard enough time determining consent and trying rape cases involving humans. Adam's argument seems to boil down to "we must treat animals like shit 100% of the time in all ways, or else outlaw all these other industries I don't like which mistreat animals." Imagine the legal shitstorm that would result.
That's not even getting into the issue of how you couldn't blanket legalize animal sex in the first place, because some species would be killed by inserting a human penis into them. You'd have to selectively legalize only for certain species (and subspecies of species), even if everything Adam has said was 100% correct and true. Then you'd also have to regulate for certain KINDS of sex vs other kinds, and then the courts would have to determine WHICH kind of sex was performed and if it was a legal kind or an illegal kind. I say you would have to do this because again, some subspecies would be medically harmed by certain types of sex, which would constitute animal abuse and even Adam agrees that should be illegal.
The end result would be writing a mountain of new laws and making the trying of animal rapists in court nearly impossible (as long as they were careful). Adam may think he's an animal lover, but believe me when I say that animal advocacy groups all across the USA would fucking hate his guts and hold him up as the gold standard of an animal abuse advocate if he made this shit legal."
Tl;dr: Zoophilia laws would make it very easy for people to sexually abuse animals, and to make such laws exist would be a legal shitstorm.