The term "sexual perversion" certainly does have an objective meaning, something along the lines of "Any act, other than heterosexual vaginal intercourse, engaged in for the purpose of sexual gratification."
Now, that's not to say that any act fitting that description is necessarily morally wrong. And while I think it's sick and disgusting, I can't see how engaging in non-physically harmful sexual acts with animals who are both non-sentient and non-sapient could be considered anything other than the equivalent of masturbation. Religious prohibitions aside, that is. (Incidentally, that leads directly to a powerful endorsement of Christianity. "If it weren't for the Bible, I wouldn't be able to formulate an ethical objection to f**king a sand shark!")
Of course, there is the rub in that we don't have any absolute method of evaluating whether or not an animal is sentient and/or sapient; maybe water buffalo were actually the first to develop analytic geometry, and then Descartes stole it from them, and cursed them to never being able to tell anyone. So to be on the safe side, it's best to stay away from bestiality, Christian or no - but that's a practical ban, not a formal one.
I really don't know where I'm going with this. I was supposed to read seventy-some pages for my econ class and then get some sleep... the former may not happen tonight/this morning.