A cow is an herbivore. A dog is a carnivore. In general, man doesn't eat other carnivorous mammals. They're considered unpalatable and lack nutritive value.

I don't think carnivorous mammals in general eat each other. We don't really care for carnivorous birds, either. Nobody is passing up KFC to go chow down on Kentucky Fried Eagle.

So, if you're eating dog, you're either dirt poor or you've got some weird point to make. In China, I think it's a combination of both. Yes, the Chinese are poor, but it's also true that dog-eating is a largely male activity that carries a certain meaning. It's a gross, masculine indulgence, like doing a big, smoky burnout at the Dairy Queen or smoking cigars.

That sort of thing is uncouth, and so is poverty. Ergo, eating dog is objectively uncouth regardless of cultural context.

(ETA I don't like dogs, though. Would hate to have one enter my home.)