THAT'S UNNATURAL!: Eugene Volokh has a good post about "unnaturalness" arguments. Hope that's not too awkward a phrase. Arguing that something is "unnatural" is purposely vague, and generally used to appeal to pseudo-religious or -emotional feelings that the speaker believes he shares with the listener. It allows them to agree on conclusions based on these feelings without having to actually deal with the details behind the feelings, on which they may disagree (different religious backgrounds, or whatever).
I think that similar arguments can be made in another realm: environmentalism. It's not "unnatural" to eat animals, or cut down trees, or dig oil and metal out of the ground. It may or may not be a good idea, and it may be wrong for other reasons, but it's not "unnatural".