(Disclaimer: I'm probably using the term "moral relativism" in a non-standard fashion. So sue me.)
I mentioned before that scientific knowledge acted as a very strongly-conserved component of the more general idea of usefulness. I'd like to throw a little light on another conserved component: enlightened self-interest.
ESI is the basis on which rationalists make moral decisions. The basic principle is simple - in order to achieve one's personal goals, it is useful to support a society in which achieving those goals is easy. If you enjoy walking in the midst of unspoiled nature, don't leave crisp packets behind you. If you aim to get fit through a swimming regime, don't piss in the pool. If you want people to be nice to you, be nice to them, and encourage them to be nice to each other.
This bears some resemblance to the Broken Windows approach to crime prevention - even small acts of personal responsibility can help create an environment that is far more enjoyable and/or effective. Likewise, exhibiting sociopathic or psychopathic behaviour (as many theists seem to believe atheists should) results in a very poor-Quality environment. We're all in the same boat - it's daft to drill holes in the bottom.
Why does this differ from absolute morality? Because the path of greatest ESI may vary between different times and places. To take the classic example: lying is not wrong when you've got Jews hiding in your cellar and the Nazis knocking on the door and asking if you've seen them. It's possible to imagine situations in which any "sin" could actually be the best thing to do in order to create a better environment. ESI morality is a means to that end, not an end in itself.
Obviously it's impossible to argue that absolute morality is immoral, because moral systems can only be evaluated in terms of other moral systems. However, I feel I can make a fairly strong case that absolute morality is extremely counterproductive. It makes reasoned debate impossible and causes concrete harm to individuals worldwide who have either a relative morality or merely a different absolute morality to those in power.
A recent example of where absolute morality can lead us is the case of the Australian Imam who raised outrage worldwide by claiming that unveiled women were like "uncovered meat", and were therefore at least partially responsible for acts of sexual violence perpetrated against them. As explanations go, that one is quite blatantly complete bollocks. It's fairly clear that the Imam merely had some a priori, morally absolute idea that it was good for women to be veiled, and sought to justify that in more rational terms.
What has got fewer column inches is the fact that his behaviour here was actually an improvement over the norm. In most theocracies, authorities don't even bother with faking up a rational explanation for why something is bad. The fact that it is Written would be enough - off with his head. Again this seems to be a particular problem for Islam - the specific issue that springs to mind is its injunction to kill apostates, which is quite frankly idiotic from the point of view of any of the goals that Islam claims to prioritise.
Absolute morality just is. You can't argue with it. You can't point out why it's stupid or mistaken or unhelpful, because all of that is beside the point - the morality is the important thing, and we humans are merely the protagonists or antagonists of Its justice. As I said before, I obviously can't prove that to be morally wrong. However, I think I'm justified in saying that, at least to me, it's an extremely scary concept.
Moral relativism, to me, means that our decisions are made with respect not to some calcified set of semi-arbitrary rules but with respect to the impact those decisions will have on our fellow human beings and, ultimately, ourselves. It means that we can be reasoned out of behaving badly, and reasoned into behaving in a way that benefits society. It means that, when something freaks us out, we actually have to come up with good reasons as to why it's wrong rather than simply turning to the nearest bigoted Holy Book*.
Whilst, as a moral relativist, it's impossible for me to say that moral relativism is good in all circumstances, I think it's fair to say that ESI is a useful approach in the overwhelming majority of cases, qualifying easily for Conserved Component status.
* I was chatting to a very nice (Christian) girl this evening who is going to have major problems with this when she tells her (extremely religious) parents that she's gay. A decent proportion of the public seem never to have learned that "yuck" does not constitute a valid moral argument.