First, it's been some months since I've had time to post in here and have had alot of things change since the last time I posted.
I took a philosophy class last year in which we explored this a little at the end of the term. Of course, in the class everyone was either dead silent or fervently refuting interest (I didn't help that there was a couple very hot girls in the class, but that's an aside that is accepted in college). I find it intriguing that everyone is so quick to label Human male to Canine Female abusive but is either willing to allow their partners to enjoy canine companionship. I don't think that either are abusive, and I seriously don't think that size is going to be a factor... unless the guy is hung like a giraffe or something. That being said, here's my two sense.
It's facinatingly intrigueing, frankly. We as sentient beings view interspecies coupling as foul, but I think that that has more to do with clarity of the species than anything; that being handed down for centuries. If you think that this is foul, should we address the Leviticus section of the bible on coupling with a human woman? I think that if my partner expressed her interest in it, I would assist her gladly.

A little more: This reminds me of a story I read when I was in high school about a woman slave who was kept in a cage late at night and forced to service her masters Dogs. At one point, they were jogging (her naked and being pulled with a chain attached to her nipple rings) and she tripped. Because the dog was with her and he saw her vulnerable, he mounted her with passerbys around. One note is that she only got one type of food... should I give you a hint or is it obvious?

Anyways, if anyone is interested in looking at it, I could prolly kind the copy of the philosophy paper I read on it. It was intriguing and slightly titalizing.