Appeal to nature
The appeal to nature, or argumentum ad naturam, is an informal logical fallacy that says that something is good because it is natural or bad because it is unnatural. It is popular among environmentalists, advocates of organic food, and other liberals.
The appeal to nature is fallacious for two reasons. First, it is not always clear what is "natural" or "unnatural." The idea that it is natural for all individuals to reproduce is falsified by the existence of eusocial insects. Second, even if something is clearly established to be natural or unnatural, that does not by itself demonstrate that the thing is good or bad. Many things occurring in nature are poisonous.
People who argue using the appeal to nature are not necessarily consistent. For example, when they use it, and someone responds with a counter-example, they may suddenly "remember" the lack of probative value of the appeal to nature.