First I'd like to say my stance: I do NOT care nor do I want to know what people do in their personal time. The thought of having sex with an animal is disgusting to me for several reasons. I also have no qualms with killing, preparing, and eating animals. I have never been hunting, but I would have no problem with doing so. (Although I do dislike wasting animals)
With that said.. I agree with the fact that it's been engrained that one is considered interspecies taboo, and the other okay. I think that another big factor for me is that animals cannot use language.. Even if body language is taken into consideration, do we ever really know if the animal is consenting or not?
Although both based on moral values.. killing to eat is a seperate topic entirely from sex.
Edit: It's known as well that dogs of the same gender humping each other is a sign of dominance, so it's not entirely sexual in nature.