Moral relativism is a good thing, or it ought to be. By all means, let's allow all ideas into the discourse! But it seems many people use moral relativism as an excuse to say: "It's useless to talk about, useless to compare, because no final absolute can be discovered!"
This is ridiculous. This is not the attitude of a rationalist, either. This is the attitude of someone pining for a moral absolute.
Relativism allows, or ought to allow, any rational person to compare competing ideas and ideals based on their actual aspects, and evaluate their relative merits. By which I mean: their merits relative to each other. Moral relativism doesn't and shouldn't mean you can't say a thing is wrong. There's nothing wrong with saying a thing is wrong, as long as you can say what's right and show why. When you lay out your basis, people will share basis to compare. The value is in comparison of real aspects, and discussion of how competing values can be weighed - not some black and white conclusion: "fire bad!"
In the post title, I deliberately chose a loaded word: "sin" - partly because I'm a dick, but let's be frank: even the most secular conception of humanism holds many things to be sins, whether they call them that or no. Skepticism, too, has its sins. Science is much sinned-against, especially by its practitioners.
To believe an unsupported and insupportable assertion is a sin against Skepticism. Skepticism can advance its case as to why it is wrong to believe unproved assertions.
To falsify data to support a hypothesis one "knows to be right" is a sin against science. Science can give its litany of reasons and examples to show why it's wrong to jump ahead of what you can establish via the scientific method: via hypothesis, experiment, falsification, and reproducible results.
None of this requires any final absolute to exist. Relativism means it is possible to compare and judge things relative to each other, without needing to reference some absolute. The principles of skepticism and science are not absolute. They are based on demonstrable benefit. Because we can see the benefit they are based on, we can tell where a given violation may hamper us.
We don't talk about movies or beer as if final absolutes are necessary. "It's useless to say this pilsner is good or bad, when we have no absolute beer ideal from which to judge!" Pish-posh. We sample a range, the wider the better, so as to have a broad basis from which to judge. We identify various merits from this range of experience, some of which may be mutually exclusive, some of which are not. Then we sample and judge between the beers themselves. Judgment is based on what attributes a given person values, and what a given beer's merits are for those attributes. The person can state what they value, and we can know what they're basing their judgment upon. Merit is no more than an ability to serve specific, identified needs. For beer, one need may be refreshment, another may be intoxication. If additional aspects and limitations such as price are brought in as relevant factors, all these considerations together will form the criteria against which we judge a beer's merit.
We're not trying to create an absolute. We're simply trying to make a judgment based on aspects and quantities that can be experienced and known. It's a rational process. A useful process. What good are absolutes? When did we ever need them? When have they ever once been of use?
We have no problem at all with comparisons and judgments based on merit in all sorts of areas. So it is, or should be, with actions and the rightness of actions. A rational person is capable of seeing what can be seen and judging between, based on merit. Based on specific, identified needs being served. Even if each separate economy advances its own view on the rightness of an act, still we can identify the needs being served and we can judge between. We can identify the effect of an act or its prohibition, and we can judge between the views on that act's morality or immorality advanced by competing moral economies.
This is what relativism gives us. Moral relativism is not some wan surrender to apathy, with the hands-waving excuse that it is not worth discussing. It was the old absolutism that was not at all worth discussing. Moral absolutism quashed discussion, moral relativism makes discussion possible.
Moral relativism is what makes it possible for us to tell right from wrong.
Moral absolutism made it possible only to say what we've been told is wrong.
Relativism opens fertile ground for strong and vigorous discourse, wherein people are not afraid to advance their idea of right, defend it with reference to demonstrable effects and basis, and advance it as superior based on specific, direct comparison with competing ideals. There is no need within moral relativism to shirk from hard scrutiny between competing ideals.
In an atmosphere of rationality and relativism, a person advancing a given thing as right must be able to say why it is right. When one person's "why" boils down to a strong foundation of greatest benefit to those most in need, and another person's "why" boils down to "Arbitrary Unprovable Being said so!"...please, don't let's say we can't compare. We have a pretty good basis for comparison, there.
From this discourse, different people will come away differently or indifferently convinced of different things. This is no cause for frustration. Discussion can be fruitful, even if not everyone becomes convinced of the same conclusion. The more we talk about what's important, the more basis we have for understanding where we differ, and the more powerfully we are able to come together where we agree. The more we find we can't come up with an effectual refutation of another person's view - even if we do not agree with it! - still, because we have tried and found we cannot reasonably refute it, we will come to respect how they can hold such a view. The more we understand and respect each other's differences, the more we can work effectively and peaceably together on all the things we know most benefit us all.
And those of us who believe that mixed in with all the ideas there are, there are some ideas that are truly the best and highest of what humanity can strive for - the more we are able to talk about what's important, the more those best ideas have a chance to come to the fore, and persuade those most open to them.
Ideas about right and wrong don't have a chance to change the world if people take the attitude that they're not worth talking about. In the absence of absolutes makes it possible to compare between. The fact that everything is relative makes it possible to compare every thing, and judge it based on how it relates to whatever good you care to claim. You are able to claim a good, when you can show it exists. You can compare two goods and say one is better, anywhere you can show how they intersect with the world others can see.