Now, I honestly couldn't care less what human women think of me, but... My butthole evolved to push stuff out, not let stuff in. But sticking stuff in it anyway has resulted in some of the most intense pleasure I've ever had, like an orgasm that just doesn't stop.
Now, dog butts are different, but the only difference I'm aware of is their rectums are a lot longer and straighter than ours. If that's the only difference, then, well... as long as they were okay with it - and I know that's extremely rare - then they'd be even more suitable for it, right?
Someone do educate me if there are additional differences; I'm actually very interested in whether dogs of either sex can enjoy receiving anal.. I've heard one anecdote of a female that insisted on anal, but I imagine that's either like winning the lottery or it's just a story.