Apologies, I should have made it clearer that I was referring to the naturalistic fallacy in it's casual sense, which denies the validity of drawing moral conclusions directly from natural facts. (I assumed that this usage was common enough that I didn't need to spell it out; that assumption was clearly false.)

Pace your second paragraph, it seemed to me that Phil was trying to do this, and others seem to have interpreted the post in this way too. But I admit that part of the vagueness in my phrasing was due to the fact that I was (and still am) having trouble figuring out exactly what Phil is trying to say.

I'm dubious of any moral claim, such as valuing vegetarianism or celibacy, that requires people to act contrary to their natures in a way that decreases their fitness.

Genetic fitness matters, because a moral system must be evolutionarily stable.

This implies that tragedies of the commons are acceptable under such moral system.

(Under the "tragedy of the commons" I mean a situation where "selfish" members of a population take advantage of a communally-maintained resource while not contributing to it, thus gaining free energy to outreproduce the "good citizens", thereby increasing the frequency of the genes for cheating and reducing the frequency of the genes for maintenance of the common resource. Any population consisting only of "good citizens" is evolutionarily unstable because it is vulnerable to an invasion of "selfish" mutations.)

Thanks for the clarification. I think I have a somewhat clearer idea what you're getting at now, but the distinction you are attempting to draw between ethics and goal attainment still seems wrongheaded to me. As the designer of a posthuman society, your ethics determine your goals; I don't see how the two are supposed to come into conflict in the way you suggest. (Maybe the demands of evolution will place constraints on the psychologies of feasible post-humans, but that's a rather different point.)

Nitpick:

An individual human can reason about it, and choose a different valuation; but they're then trying to act in a way inconsistent with their psychology

As it's currently stated, I don't think this claim makes any sense. If they can do it, then it's consistent with their psychology.

Why must a moral system be evolutionarily stable?

I'm dubious of any moral claim, such as valuing vegetarianism or celibacy, that requires people to act contrary to their natures in a way that decreases their fitness. [...] but when you then conclude that lions are evil for eating zebras, you've gone wrong somehow. I don't buy the argument that the rules are different for humans because they can reflect on the rules.

(Emphases mine). Please settle on a set of applicable subjects. I'm with you that moral claims apply to "people". I don't think many here will claim that humans exhaust that space in theory, or that lions inhabit it currently.

This does help clarify things. Unfortunately, Conchis is right, you're committing the naturalistic fallacy.

Your post was poorly received because it tackled a confusing topic, and failed to bring clarity. All of the supposed counter-arguments are just confabulations. I expect Less Wrongers to jump on any attempt to analyze morality in abstract terms, regardless of its conclusion, because there's an extensive body of philosophical literature showing that such attempts produce only concentrated confusion.

Also, you shouldn't expect different individuals within a community to all advocate consistent positions. If they did, that would mean that either the question was an easy one and not worthy of further discussion, or the community was broken and suffering from groupthink.

I agree with jimrandomh. One should expect different people to have different opinions. Furthermore, we're most likely to respond to things that we find obviously wrong - thus, where the community is not in consensus, expect to be 'jumped on' no matter which position you advocate.

-a moral realist of sorts