I just have to say, I've been giving this a bit of thought (unfortunately) and something odd occurred to me.
My old dog used to try to hump about half the people that came over. We kept trying to teach him not to, as both of us and everyone we knew didn't like him doing that, but he was definitely a horny fucker and always tried to get away with it anyway.
Now, if at some point I had learned that someone my dog was trying to hump actually let him, because that person enjoyed the experience and got off on it, I would of course be very grossed out and suspect trauma in the persons past. I would believe the person needs help. I agree wholeheartedly that such is bad. That said, however, I would have a hard time considering my pet to have been raped, considering that would probably make his whole day, and he'd probably have completely forgot about it the next.
The point is moot, however, because such would damage the human. The dog? I'm not so sure.
Doesn't matter what the dog wants, just like it doesn't matter if a drunk person at a party wants sex. If a drunk girl comes onto you at a party, you swat that shit away like a horsefly (Not an actual physical swat, mind you. A gentle "You're drunk. We can talk, but that's as far as we're going tonight" will do).
And that's bullshit too. It's the law, but it's still bullshit. If you're somewhere that does not have a no-true-scotsman definition of consent there's no reason whatsoever for this course of action (at least no reason that isn't a strawman that automatically assumes out of hand that you automatically won't use any sort of protection in this situation, or that everybody will automatically hear about it (and that they'll care, and judge one or both partners, which is a moral failing on their end, not on the end of the people having the sex), when neither of these things might happen otherwise or that some kind of strong sexual imprinting is likely to inappropriately take place despite the negative correlation between drunkenness and memory-formation)
EDIT:
Not that it's a good idea, mind you. There's the usual, perhaps even a more than usual (but still very far from definite) chance of doing something that could result in a chronic disease or an unplanned pregnancy (unless it's homosexual sex, which can never result in an unplanned pregnancy). I just take exception to the idea that this would be much worse than (or indeed, even close to as bad as), for example, letting this person have their car keys, which could cause a car accident. The worst case scenario for a car accident is people, perhaps even many people, being crippled or killed - very few people get killed outright by sex, HIV has been reduced to a chronic illness and to my knowledge there aren't yet any superbug strains of syphilis (gonorrhea yes, but that doesn't kill you and should clear up in under a year even without treatment) - and even an accident where nobody's hurt could mean expensive car damage, expensive damage to anything they drive into, and/or hundreds of people inconvenienced by accident-related traffic. (And yes, they could also cause an accident while sober, but so too they could also get an std, unplanned pregnancy, or reputation for sluttiness while sober)
