Zoosexuality is a sexual orientation where people have a sexual attraction to animals that they did not choose. It's sometimes also called called zoophilia, but that could also mean being kind to animals without doing anything sexual. Many zoos care a lot about animals and many abstain from having sex with them. Rural conservatives are sometimes supsected to engage in acts of bestiality, which many have committed.
For and against
It's been said that in the same way that we should tolerate, treat fairly, and otherwise not oppress LGBT people, we should not oppress people who engage in bestiality. We don't advocate it, we just say people shouldn't be oppressed.
- Opponents allege, the difference can be said to be one of consent. Animals are not able to give full and informed consent for sexual encounters, because they are not likely to be fully aware of what is happening and it's implications. Even if they were able to consider the situation fully, they may not be able to fully express their opposition to the act. Because of these factors, animals can be considered to be non-consensual by default in much the same way that children are.
- Supporters point out that animals don't always consent when animal breeders mate them to partners humans have chosen. Animals don't consent to being sold to new owners, often this involves separating a young animals like Puppies and Kittens from their mothers and there are unlikely to be checks whether the new owner will be kind. Supporters also point out that the way they treat their animals is no worse than the way some Conservatives treat their chattel wives and slaves. Animals don't consent to being slaughtered and turned into meat though anmials are often unecesarily stressed during slaughter. For supporters concern about animal consent over sex with humans only while other areas are neglected is inconsistent. It looks a bit like a rationalisation for the way some humans dislike zoosexuality. 
- Progressives may be less than willing active supporters. With all the important changes needed in our society, it's tough to justify spending political capital on this issue. Yes, we're Liberals, and we're for equality before the law, but "Damn Dude, do we have to go there?"
If there is sadism or callous cruelty to the animal then it should of course be illegal under animal cruelty laws.