"You are committing the naturalistic fallacy!"
Unfortunately a number of distinct things have been called 'naturalistic fallacy' which aren't what G.E. Moore, the fellow who coined the term, originally meant. Here are some things which aren't the naturalistic fallacy.
There are two fundamentally different types of statement: statements of fact which describe the way that the world is, and statements of value which describe the way that the world ought to be. The naturalistic fallacy is the alleged fallacy of inferring a statement of the latter kind from a statement of the former kind.1It was Hume, not Moore, who pointed out that arguments from what is the case to what ought to be the case must include a step that connects the former to the latter. This is commonly called the 'Is-Ought Problem,' though it should really be called the 'Is-Ought Reminder.'
The fight against the naturalistic fallacy was supposed to be over. One of the great achievements of modern philosophy was to undermine arguments from Nature. No longer would educated people argue that homosexuality, powered flight, and the education of women were unnatural.2I don't support the idea that whatever is 'natural' is good and whatever is 'artificial' or 'unnatural' is bad (or at least suspicious), as I'm quite fond of some unnatural technology and avoid natural poison whenever I can!
But this isn't the meaning of 'naturalistic fallacy' in modern philosophy.
Do not fall victim to the naturalistic fallacy. Simply because something evolved does not mean that it is right or justifiable. Nor does it mean that it cannot be changed.3A common variation, in which evolution rather than traditional prejudice defines 'natural' and 'unnatural.'
The Genuine Article
What Professor Moore means by the 'naturalistic fallacy' is the assumption that because some quality or combination of qualities invariably and necessarily accompanies the quality of goodness, or is invariably and necessarily accompanied by it, or both, this quality or combination of qualities is identical with goodness.What's most striking about this genuine definition of 'naturalistic fallacy' is that any of the above impostors can be invoked without committing the naturalistic fallacy! For example, it could turn out to be the case that whatever naturally evolved is also good and nothing is good unless it has naturally evolved. We could then make correct judgments about goodness by checking whether a thing has naturally evolved or not. And, so long as we don't claim 'naturally evolved' and 'good' must therefore be one and the same quality, we avoid the naturalistic fallacy.
If, for example, it is believed that whatever is pleasant is and must be good, or that whatever is good is and must be pleasant, or both, it is committing the naturalistic fallacy to infer from this that goodness and pleasantness are one and the same quality.4
Another striking difference is that Moore's point concerned metaethics, while impostors tend to focus on normative ethics. Moore did have a lot to say about normative ethics, but he didn't write off other views as necessarily fallacious. Instead, he claimed that advocates of those other views tend to commit the (genuine) naturalistic fallacy and so they might reconsider their views once they realize this fault:
Now, I do not wish the importance I assign to this fallacy to be misunderstood. The discovery of it does not at all refute Bentham’s contention that greatest happiness is the proper end of human action, if that be understood as an ethical proposition, as he undoubtedly intended it. That principle may be true all the same; we shall consider whether it is so in the succeeding chapters. Bentham might have maintained it, as Prof. Sidgwick does, even if the fallacy had been pointed out to him. What I am maintaining is that the reasons which he actually gives for his ethical proposition are fallacious ones so far as they consist in a definition of right. What I suggest is that he did not perceive them to be fallacious; that, if he had done so, he would have been led to seek for other reasons in support of his Utilitarianism; and that, had he sought for other reasons, he might have found none which he thought to be sufficient. In that case he would have changed his whole system—a most important consequence.5So, like Hume, Moore is often misunderstood as claiming whole classes of ethical views are invalid. Both of them were really just highlighting a consideration moral philosophers should take into account which had previously been overlooked.
1. from http://www.logicalfallacies.info/relevance/naturalistic/
2. from http://www.law.duke.edu/boylesite/tls98nat.htm
3. Davis, S.F., & Buskist, W. (2008). 21st century psychology: a reference handbook. London: SAGE Publications, Ltd. p. 259
4. from http://www.utilitarian.org/texts/prior.html
5. from http://en.wikisource.org/wiki/Principia_Ethica/Chapter_I