Naturalistic fallacy

The naturalistic fallacy or appeal to nature is a logical fallacy that is committed whenever an argument attempts to derive what is good from what is natural. Originally it was considered a type of equivocation, wherein the word "good" was used in the sense of "pleasant" or "effective" in the premises, and in the sense of "moral" or "ethical" in the conclusion. Now it refers to any case in which someone refers to something as morally necessary simply because it is more natural.

The converse argument, where one assumes that whatever is good must be part of the natural order, is known as the "moralistic fallacy".

Examples
A trivial example:
 * Apples are good to eat (meaning they are delicious or have nutritional value).
 * Therefore people who eat apples are better people (meaning more ethical).

A common Christian argument:
 * Homosexuality is unnatural (meaning against the biological human drive to procreate or against the supposedly God-given purpose of sex).
 * Therefore homosexuality is wrong (ethically).
 * Note: the first premise here seems to be untrue, based on scientific investigation into the causes of homosexuality. The causes are not understood but natural biological processes are known to be factors in at least some cases.

Social Darwinism:
 * Natural selection works because the weak/stupid/disabled die and the rest survive to reproduce.
 * The weak/stupid/disabled should therefore be allowed to die or killed to keep the process going.

"Ought" vs. "Is"
As David Hume pointed out, knowing about the physical world never, by itself, tells you how to behave ethically. To behave ethically, you must not only know what is out there, but also have a value system that tells you what ought to be out there and what your place ought to be in making that happen. Knowing about the natural world is not enough to build a moral system.

"Natural" can mean many things, and none of them are the same as "moral"
"Natural" as opposed to "artificial" is not the same as moral, or else we would be obliged to give up sanitation, penicillin, modern agriculture, and other life-saving inventions as "immoral".

"Natural" as opposed to "supernatural" is not the same as moral. For one, many theists believe in both evil supernatural beings like demons or malevolent spirits and in good supernatural beings like gods or angels. For another, many people who believe in no supernatural phenomena at all have detailed moral codes.

"Natural" meaning "biological" as opposed to "cultural" is not the same as moral. Both altruism and psychopathy seem to have roots in human physiology. Similarly cultural influences can be either good or bad.

Natural law
Natural law can refer to many different philosophies. Generally natural law philosophy simply posits that there is some objective set of ideal rules describing ethics, which all human beings should theoretically be able to agree upon and derive from a rational understanding of the world. Such philosophies usually start with some very small number of broad values (like "Happiness is the ultimate good." or "The interests of every person should be weighed equally.") and attempt to derive an entire system of ethics from them.

Some natural law philosophies additionally state that the "natural law" was created by God. In these philosophies, it is not a fallacy to claim that "natural" and "moral" are the same, because both were created by God to be consistent with each other. However, these philosophies are only viable if one can demonstrate the existence of an objective moral law and a God that created both the natural world and that law.

Strikingly, even in this kind of natural law, it is fairly useless to say that something is wrong because it is unnatural. These philosophies define both what is "natural" and what is "good" as being whatever is in accordance with God's will towards creation. To prove that something is unnatural in this sense means to prove that it is against God's plan. But "natural" already has straightforward definitions. To add this other, more unusual definition serves no purpose except to confuse the issue and risk some form of equivocation (between a usual definition of "natural" and this unusual meaning of "according to God's plan"). One might as well ditch the word "natural" entirely and use a different word or phrase instead.