So... Why do you get mounted by male dogs? That's also UNNATURAL! Shame on you! XD
Following your logic, only male humans on female dogs (or male k9s on female humans) should be accepted, because it's the CLOSEST thing to what happens more often in nature...
Yeah, I don't believe myself that many dogs enjoy anal, as I don't believe that many bitches enjoy vaginal penetration (I've seen lots of vids where they looked annoyed or even tried to run away, I've seen mating vids where bitches were clearly in pain, so I've never believed to the Fairy tale that sex is ALWAYS pleasurable).
I think that we should just be thankful that our dogs accept us as their partners, and even more thankful when they let us have sex with them outside of heat (other thing that doesn't happen in nature xD), whether it is vaginal or anal...