To my best understanding, it's their requirements for conscious sentience.
Basically, the same reason screwing someone with downs syndrome is frowned upon.
I, personally, go based on sexual identity.
If it could theoretically rape, it can be raped. You can't rape a chair because it lacks the ability to hold an identity sexually. Identities can be as simple as "I am ___. I like sex. Sex feels good."
If they're capable of recognizing the above and are mature physically and mentally, I see no issue.
Other people don't consider sexual identity, so when they think of bestiality, they treat it like they do with mentally stunted people.
Basically, the same reason screwing someone with downs syndrome is frowned upon.
I, personally, go based on sexual identity.
If it could theoretically rape, it can be raped. You can't rape a chair because it lacks the ability to hold an identity sexually. Identities can be as simple as "I am ___. I like sex. Sex feels good."
If they're capable of recognizing the above and are mature physically and mentally, I see no issue.
Other people don't consider sexual identity, so when they think of bestiality, they treat it like they do with mentally stunted people.