Sweary historian Eleanor Janega writes on her Going Medieval blog (previously) that there was never a time in medieval Europe when bestiality was socially acceptable, and brings the receipts in the form of eyewatering details on the punishments for having sex with animals.
Janega mentions that, of course, any non-procreative sex was doctrinally suspect in the Christian tradition of the day, because sex was what got humankind kicked out of Eden, so the only time you should be getting it on is to make babies, otherwise, you're just celebrating the stuff that makes God angriest.
But sex with animals was especially bad for medieval theological authorities, because it was "against nature" in that it "humans are blurring the clear line between themselves, as beings with a consciousness, and animals who are a separate category without the faculties to reason."
But here is the kicker – one of the reasons that bestiality is considered so terrible is because animals cannot reason and therefore cannot consent to it. Animals, famously, were not cursed by God for eating forbidden fruit. They are still therefore running around most nakedly. Because of their innocence, they also mostly just have sex for the purposes of procreation. Now, there are some notable exceptions to the whole sex for procreation thing in the animal kingdom, most notably our good friends and group sex enthusiasts the bonobos, but also dolphins and orcas, and all the gay animals, just off the top of my head. Everyone else though? Mostly having sex in order to have babies and not thinking about it at all. Animals therefore do not want to have sex with your weird ass because they only really want to have sex in order to make babies and you can't do that for them. So, if a person decides to have sex with an animal they are doing it only for themselves in order to do something sinful and despoiling an innocent. They are turning their back on their own rational nature and taking it out on animals that have no recourse to rationality, and yet abstains from sex except for an approved reason.
No bestiality was never OK, you absolute rabid weirdo [Eleanor Janega/Going Medieval]