I think anyone who eats animals-and thinks about eating animals-is at least somewhat cognizant that the choice to do so is, on some level, an ethical one. Of course thoughtful meat-eaters are not walking around with their noses buried in Bentham, but they do, by virtue of being thinking meat-eaters, at least entertain the idea that there's a basic difference between eating a pork chop and a piece of toast. A moral difference, no less. Put simply, for anyone who is honest with himself about the decision to raise and kill animals for food we don't need, there's a vague idea that eating animals under certain circumstances might very well be morally wrong.
It all comes down to the realization that an animal, like us, has interests-the most basic of which is avoiding pain. Because we cannot, as decent people, go through life thinking that our interests matter more than other interests simply because they are ours, we thus tacitly grant to other humans and many non-humans-basically anyone with an interest in avoiding pain and seeking pleasure-what philosophers call equal moral consideration. We may not even be aware that we live our daily lives according to this standard but, in most cases, we do. We often just call it the Golden Rule or some such and get on with the business of being decent folk.