Everything that exists in the universe is inherently natural. It would be ridiculous to label something as "unnatural" is everything around us exists in nature by default.
For example, one of the main arguments in regards to something "natural" is homosexuality. Many religious people label homosexual behavior as "unnatural" based on bias religious propaganda, not based on a realistic, accurate definition of what nature is. In order to be considered "unnatural" something must defy the natural laws of science and reality. Homosexuality is apart of nature and apart of life for most animal species and because it exists, it is natural.
Nature is everything. What is there that isn't natural? Mutations? They have always been necessary for nature to continue. Evil human actions? They have always been inspired by instincts and false concepts, neither of which are unnatural. Take Hitler, for example. He killed many Jews, and many other people, based on the false concept that they were evil, despicable, etc. And that God wanted them dead.
Forest fires, hurricanes, tornadoes. Those are natural. Even wars, bombs, and torture those things are also natural because they take place as parts of natural processes.
Hence natural/unnatural distinctions are meaningless and using "nature" as a guide to morality is absurd. The argument "it's not natural" is a meaningless wasteful argument.
As long as you can say: "by nature, I mean...", then it is a perfectly valid concept. If I assert that gays are against nature, and define nature as "that which we observe in other life forms", than you can refute me by stating that homosexual behavior has been exhibited in other life forms.