The consensus so far is that I’m wrong. Two emails:

I think you’re wrong.
To call it a naturalist fallacy suggests that, without the observations of relativity, relativism would not have happened.
This is certainly quite false.
First, the idea of the relativity of morals, knowledge, judgment, etc. was heavily propounded in the late 19th Century particularly in Germany long before Einstein. Nietzsche, and the entire Neo-Kantian school picked up on this. (Ernst Cassirer is a good late neo-Kantian.) Poincare implied the same thing in his popular mathematical writings in France.
It’s important to understand that people did not go out of their way to “make this stuff up” — all they were doing was following the logical consequences of secularism, which disenthralled the traditional absolutes derived from religion, and following the the consequences of the “platform problem” that all philosophies then have (as Hume recognized from an empirical standpoint, and as the neo-Kantians did from an idealist standpoint.)
Einstein’s theory — which became most popular _after_ World War One — simply became a focal point for popular understanding of a process of relativization (or non-absolutization) that had already been going on for many years.

And, from a Philosophy Prof:

Jonah,
You’re wrong. The naturalistic fallacy is the fallacy of identifying the
property of goodness with some natural property (such as conduciveness to
survival, pleasantness, whatever). It’s not really a fallacy in the strict
sense, that is, an invalid inference. But G. E. Moore got a lot of mileage
out of accusing those who disagreed with him of committing a fallacy. (If
he hadn’t called it a fallacy, he would have been reduced to saying, “You
have committed the mistake of disagreeing with my view,” which has rather
less oomph to it.)
I suppose you could use the term more broadly to designate any inference
from the-way-the-natural-world-is to some purported ethical truth. But to
say that any such inference is invalid is to take a huge substantive view
within ethics, one that itself needs considerable defense.
Thomas