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.