Hi, Don!
Just in order to clarify why it is considered a fallacy, think about this exaggerated example. Imagine that, in a community, people believe that men should work and women should stay at home. They might say: "for generations things have been like this, it's the natural order, it must be like this". Philosophers say that this reasoning is wrong, because they are infering a normative proposition from factual propositions. They're infering an ought from an is.
Curiously, the recognition of this kind of problematic reasonings is found first in David Hume. Moore took it and developed it in order to defend his idealistic theory of the Good.
Now, for more clarification, the paragraph you quote form Wikipedia takes the metaethical sense of the naturalistic fallacy. The basic idea is that in a naturalistic ethical project (like the Epicurean one, for example), holding that pleasure (or any other natural entity) is good, implies that there's something in pleasure that makes it good. The question is, which property is that and why pleasure has it? For them, it implies that pleasure is a privileged entity, because it has the property of being ethically good. Here, people like Pigliucci may say that pleasure has an evolutionary and instrumental origin, so it can't have the privilege of being the entity with the property of THE Good.
As you may see, this approach looks suspicious, because is taking the naturalistic Epicurean theory in a kind of platonic terms. And then, for surprise of no one, falsify it.