zoophilia is wrong if the recipient doesn't give consent
obviously you have no way of knowing if the sheep/donkey/chicken is happy to be fucked, so it's wrong for the same reason it's wrong to fuck a spastic or a vegetable-person
if the non-human animal is the one doing the fucking though, then it's ok
consent is the issue with all sexual things: if all parties consent, there is no problem (assuming you take consent to imply adult)
obviously i have never fucked my cat, my tortoise, my dog, my gecko or my finches
i never fucked my hamsters or my rabbit either, even though my rabbit regularly tried to fuck me and my cat