I've been thinking about your initial post since I first read it. I had never before thought about the animal's perspective. If I personally weren't so grossed out by the idea (just my personal preference folks) I may have stopped and thought about it. However, the more it sits with me, the more I have to agree with you.

Safe, sane and consensual is an automatic given for people, but what of the animal? You are correct than an animal cannot give real consent. Granted in a sense, it's cooperation could be taken for consent. But in that scenario, realistically I can only see the animal doing that if it is trained to do so. Yes we train dogs to fetch papers, horses to pull carts, animals to do circus acts. But honestly, those aren't really comparable acts to sex.

But I can't really think that teaching an animal to take sex from (or give sex to) people is something they can really understand to give their consent to. Nor can we really measure their level of comprehension of the event.

As to whether bestiality is part of bdsm or a fetish? I am going to have to put it in the fetish pile. As with any fetish it can be pulled into a bdsm scene, but I don't think it is part of the core of bdsm in itself.