It doesn't justify anything. Not saying that meat eating, polyandry, and homosexuality aren't justifiable, I personally don't have a problem with any of those things.
And what is natural anyway? There isn't really an objective definition of it. Calling something natural is implying that such a thing is supposed to happen, without force. The only reason we really call my things our species does 'unnatural' is because we are sentient beings, and thus we have the ability to perceive our actions as forceful. We destroy nature, and we consciously do it, for reasons that we can consciously realize. But nature can do the same, it just lacks the sentience behind it, it just is, and so are we. The difference is, we're aware of ourselves.