Yeah, animals can try to give that consent, but humans cannot accept it meaningfully any more than an adult can meaningfully or ethically accept clear advances from a child.
I see it as a different situation with animals--we can legally kill our animals. We can arrange for them to have sex with their own species, or we can deny them that ability. Why can't we make that one decision?
I think the applicable law should be cruelty to animals. You harm the animal in the process, it's illegal. No harm done, it's just yuck. I do not believe the state should prohibit private yuck.
And on some levels, I agree with you. I'm fact, when I joined these forums, I made similar arguments. The problem with it is that it's really hard to walk that line: people, and animals too, get in a lot of really hairy (furry?) situations when sex gets into the mix. At some point it's hard to separate out your wants from their needs and they don't exactly have mouths or even minds capable of getting that deep into it.
At this point I think that a lot has to do with a particular cognitive dissonance that happens with pets: we pretty much assume they aren't technically "people" but we treat them like "people" nonetheless. It just seems a bad idea to think that you should be able to manipulate and groom something that can't really ever get away from you for sexual purposes. The implications of someone who could do that are really fucking bad.
Everyone who I've ever known who HAS done that (yes, I've encountered a few...) is also the sort who engages in abusive or coercive relationships.
In reality, we have to ask the question of what particular form that harm takes; to me, it is the harm of the decay this causes to normal consent behaviors.