Animals cannot agree to sex with a human - but I'm skeptical that it actually harms them unless there's physical damage. They didn't consent, but the ones that were kept locked up for their entire lives and then killed didn't consent either. And the majority of people take no issue with the latter lack of consent.

I think there's a cultural quirk about sex - it's on a pedestal, so much that wielding it improperly is a crime worse than murder. It's one of the very small number of things that non-adults can't consent to. In the case of human victims there's clearly harm being done, but with say, a horse? I don't get why people are into it, but I'm inclined to let them carry on so long as no living being is harmed.