I'm in the boat that just because animals can't speak english doesn't mean they can't give consent (no, I have never fucked an animal). Dogs are pretty smart, and they know when they're being harmed or when they don't like something. If you start trying to do sexual shit with a dog, and it's looking uncomfortable and trying to get away, then it obviously wants no part of the situation, and if you keep pushing it, then that's wrong. But if you and some dog are fucking, and it hops up and mounts you very enthusiastically, and keeps going for it even if you pull away, then I'd say it's enjoying itself.
People are the same way. They don't always explicitly say "Yes, I would like to have sex", or "No, get the fuck away from me". But if you're making advances on some chick, and she's obviously uncomfortable, you need to stop. But if she starts riding you and won't let you pull out or anything, it's probably a safe bet she's enjoying herself. No words were exchanged in either scenario, but it's still pretty obvious whether she consents or not.
So I really don't see why it matters who or what you're having sex with (children being the exception), as long as everyone is having fun.
But that's just my two cents.
