Or maybe it's the belief that humans have abused, and lost, their license to harvest pigs for meat. Take a look at the conditions pigs are raised on in commercial farms. Locking any animal in a cage for its whole life, where it cannot move or tend to itself, is inexcusable. It doesn't matter how intelligent it is, it would be just as impermissible if it were a bee trapped in a cage the size of its own self.

Being delicious is a great adaptation though, although it dooms individuals to a premature death, it has secured the future of the species. As long as humans are around, pigs and cows will be as well.

If we weren't so keen on describing things involving humans as "unnatural", I think the relationship between humans and livestock animals would generally be seen as symbiotic (much like ants who farm fungus).

I hear humans taste like pork. Cannabalism hasn't really taken off though.