Progressives run from principles of relativity because they believe that these are principles of... well, I would call it nihilism. I.e. "moral relativity" is held to be an idea by which anything goes... that your moral judgments are totally local.
I disagree, while the symbol which is the two words "moral relativity" can be assigned whatever real phenomenon one likes, I draw my idea of relativity from the physical sciences. It's not just from physical theories, such as General Relativity, but more from the principles of relativity which Einstein, for example, used to deduce from thin air his theory of Special Relativity... this is a principle also evident in the so called classical model of Newtonian dynamics which contains, for examples, velocity relativity.
This latter type of relativity is the more or less widely known relativity that says you cannot know a body's velocity by itself, but only in relation to another body... if two spaceships are traveling toward one another at 1000 mph, this is indistinguishable from one of the two being motionless and the other traveling toward it at 2000 mph.
It is a principle of relativity that there is no privileged point of view, however, this does not mean all things are equal, ironically, it means the opposite. No two things are equal (or else they would be one thing). It means all you really can do is judge relationships, and this idea applies to social and moral life.
In short, it says you cannot construct final judgment for a "thing in itself", but you can judge the relationships, where "judging" is the human analog of a physical measurement... a human moral measurement... which applies.
Here is an example, I take a difficult example so as to keep it interesting, I hope.
The Slave Owner. What a despicable thing slave owning is. Yes? We would like to say, the slave owner is, thing in itself, an evil person, an immoral person, probably unredeemable. But relativity says that we do not judge the master in isolation or the slave in isolation, but it is their relationship we are able to judge, to judge as immoral.
Is that acceptable? Well, consider the case of Thomas Jefferson, slave owner. Few would say that Jefferson himself was unredeemed, instead, his apologists often claim that "things were different"... and this somehow forgives Jefferson. I never found that appealing, yet I do respect Jefferson at the same time, but not for slave holding. I believe relativity provides a better explanation than merely forgiving him due to what was "acceptable" at the time, not least of which because some at the time DID NOT accept that slave holding was morally legitimate, and they were RIGHT AT THE TIME. They were right at the time because the relationship between slave and master is bad, or so I and most people judge, and it does count against Jefferson, end of story. Had Jefferson repudiated slavery, in other words, he would have been even greater, great in yet another domain besides architecture and democratic philosophy.
Slavery is not justified, "back then", by virtue of it being common and accepted. It was wrong it always was wrong. We fight slavery because it is wrong, and it is a relationship between humans which we discover by studying the relationship between master and slave, a relationship of starkly uneven footing. Some still find that a fine relationship, and skepticism, more than relativity takes that into account (for another time, skepticism includes an acceptance of what is apparent, over dogmatic assertions, and various demonstrations that what is apparent is, ironically, much more compelling than dogmatic assertions)
We judge slavery as wrong and immoral (those of us that do) because of the elements of injustice in the relationship, but not because the slave owner was
immoral-in-himself and lent this immorality to his actions, as the classical analysis argues.
This doesn't change the real problem, and of course, it's still open to debate after a relativistic analysis, whether or not Jefferson should be characterized by that relation to the world, or by some other relation, say his relation to Americans as one of the founding fathers. One relation is characterized by robbed liberty, the other, by liberty granted.
This may seem to do nothing but rearrange some semantics... but there is more to it than that.
It explains how we can call Jefferson liberal, though he was a slaveholder which is not a liberal institution. It focuses us away from the difficult (or impossible) question of intrinsic worth or character. It changes our perspective from one that tries to judge a man great or foul, and expects purely great or foul accomplishments to follow, into one where we analyze an individual's relationships to their nation, to history, to other individuals. It explains better the imperfections of humans and why we take instruction from these imperfect humans nevertheless.
To try another difficult example in brief... Hitler is thought of as an evil man, and I concur with that because his relation to his world was so broadly dominated by relations easily judged as immoral and intentional to the point of "evil", from his propagandistic treatment of his fellow Germans, to his preference for war, to, of course, his relationship to the victims of his genocidal and murderous tendencies.
Yet none of those relationship, dominating our judgment of Hitler, should confuse us into thinking that the VW Beetle was also a work of evil merely because it was the result of a Nazi commission... in that case, providing a cheap car for the German people, we have a lone relationship which can be studied in isolation, which survived in isolation, which outlived the Nazis, and which, as much as Hitler's mere presence in the equation fouls the history of that car, didn't stop it from eventually becoming a symbol of something quite distinct from Nazism. A relativistic approach explains this, because in this relationship, Hitler to the Germans that benefited from this inexpensive and easily maintained car, the car is good, and insofar as that car embodied this relationship to it's driver, it was a benefit unmolested by a regrettable association.
This extra example muddies the water but I wanted to remind you before the essay's end that this is
not a way of considering Jefferson, this is a general way of judging
history, men, and reality. While the relativist says you cannot judge things in isolation, s/he
does not say you cannot judge at all, but redirects your facilities to
judging the relationships between things, which you may still do according to your own intuition and as those
things relations appear to you.
updates: for typos and wrong words only