What I'm saying is that to argue that our ancestors were sexual omnivores is no more a criticism of monogamy than to argue that our ancestors were dietary omnivores is a criticism of vegetarianism.
Our ancestors were dietary omnivores. As a result, we are optimized for that diet, and while that doesn't mean that it's the best possible diet, it is a good reason to believe that you're better off eating some meat. If you're just concerned about you, that's a pretty good criticism of vegetarianism. If you're concerned about animals, or you believe that a god you trust told you not to eat meat, or something like that, it's still a valid criticism, but it's a lot less important and it doesn't address the main reason you chose to be a vegetarian.
Another month has passed and here is a new rationality quotes thread. The usual rules are: