This rhetorical device - that it's ok to do x because x is natural - is called an "Appeal to Nature". At base, it argues that because something's natural, it's good. You can use it to justify all sorts of things - rape, murder, sticking your dick into anything that moves - all manner of things. One of the nice things about being human is that we're now in a position to choose whether or not to live by following our natural desires. And in many cases, we've chosen not to - we sanction against rape, murder etc, and many, many cultures, we eschew eating meat. So the argument really isn't whether we should eat meat because it's "natural" to do so, but rather, given we don't really need to, should we eat meat. "we should eat meat because it's 'natural' to" isn't really much of an argument.
