The New York Times has an article this morning ("Economists Dissect the ‘Yuck’ Factor") about the role that disgust plays in moral intuitions. We've evolved to judge that things are wrong if they feel unnatural somehow.
The article goes into how disgust affects economics. According to Yale psychologist Paul Bloom, money still feels unnatural because it's a historically recent innovation, which means that we feel fine trading human organs, but not purchasing them.
The article also talks about the role of culture in shaping morals: "In Europe and Japan horse meat on a menu would stir no more comment than macaroni and cheese would in an American diner." Brings to mind a piece PT did a few years back: "Is it wrong to eat your dog?" The answer: That depends on where you live.
And how hungry you are.