This is an expression of the is/ought gap which separates objective facts (what is) from ethical behavior (what ought to be). The theories of nature and sciences only provide a description of how the world works, not a prescription for how people should behave. On the other hand, ethics deals not with what is scientifically true, but what outcome is good and desirable. Hence using science to justify ethics is impossible because, pragmatically speaking, ethics cannot be tested in nature.
Unlike the laws of physics, the laws of morality can be disobeyed, such that science cannot prove the transgression to be inherently wrong. Science only knows that murder exists, and cannot prove murder is wrong (this often comes up in questions of What Is Evil?). Science cannot answer questions of good and evil, these being out of its jurisdiction. All science can do is inform ethical decisions by telling people how various aspects of nature work, such as whether smoking tobacco is harmful and thus it may be immoral to advertise to people one shouldn't be harming (there is a minority of philosophers that disagree, saying science can determine moral values).
It can also arise from a fallacy of ambiguity since the words "normal" and "natural" can both refer either to "what is commonly done" or "what should be done" in given situations.
Still, when making a moral argument for or against a given practice, not providing any evidence whatsoever that this will have a desirable or undesirable effect (making an ontological argument or engaging in Circular Reasoning) typically gets one denounced as a liar and a charlatan. This leads to widespread use of the latest scientific discoveries to justify various behavioral or moral standpoints.
In politico-religious discussion by Moral Guardians, the contention often comes up that "Homosexuality is im/moral because it does/does not occur in nature." Unless (as some animists and pantheists believe) nature is a sentient entity capable of making its own decisions, these arguments are inherently meaningless and futile. (And many of these arguments are completely wrong (for instance, homosexuality does occur in nature).)
Naturally, people who are opposed to using nature to justify a moral standpoint will claim that Nature Is Not Nice, that it's rife with disease, natural disasters, parasites, predators, murder, rape (arguably, depending on how one views certain animals' mating practicesnote ), and other ghastly things, while on the other hand, "unnatural" civilization gives people the means to behave more morally, to conceive and develop ethical philosophy, and dramatically lower the human mortality rates. Those who take this position also have the convenience of questioning at what point in pursuing their natural desires (such as the hunger and procreation drives) humans' activity stops being natural and starts being artificial; this is not an easy distinction to make, as all resources and technology, no matter how sophisticated and derived from human planning and decisions, is still bound to operate according to the laws of nature.
Furthermore, criteria for determining what's "natural" and what's "artificial" vary widely: some might say that which is created or done by humans is not natural, and everything else is. Yet beavers create dams—are they unnatural? What magical ability do humans have which somehow corrupts everything they construct from nature's resources? Whatever distinctions one draws, humans are still necessarily part of nature, possessing animal needs and desires just like beavers. Therefore everything humanity creates is by definition also a part of nature. Apart from some kind of supernatural intervention (e.g. food being created by miracles), nothing that exists can unreservedly be said to be unnatural!
Moral Guardians are not the only ones to employ this fallacy; for example, some despotic ideologues have made arguments from natural selection to justify Social Darwinism and the sociopathic extermination of those they deem "unfit" or "unworthy" of life. Another extreme example is Straw Nihilists who contend for unbridled hedonism on the grounds that we are nothing but animals pursuing our animal desires for food, shelter, and procreation and should therefore stop "denying" ourselves these "healthy" natural pursuits. It is also common for people arguing against whatever view is being presented in this manner to fall into this fallacy by arguing that the thing they are defending is in fact natural as a way to "prove" that it is moral or acceptable — for instance, arguing against a nihilist viewpoint not by attacking the claim that we should structure society around "natural" drives but by trying to argue for morality or cooperation in nature.
In matters of ethical philosophy, ethical non-naturalism, existentialism, and other presuppositional moral codes attempt to defy this trope, but are often unpopular because as noted, the abstract presuppositions on which they rest cannot find any support in concrete scientific observations.
This fallacy is related to the notion that Science Is Bad, and frequently underlies the advertising pitches for All-Natural Snake Oil. An Alien Non-Interference Clause is partially an attempt to enforce this belief on ourselves in the event of contact with other as-yet unknown (usually extraterrestrial) sentient species on the grounds that interfering with their civilization's "natural" development would be wrong. This may also be related to The Farmer and the Viper, if someone asserts "it's in my nature" as an excuse/justification for an evil act. A specific and more contemporary outgrowth of it is the belief that New Media Are Evil.
