To understand how post-modernism can avoid absolute relativism in ethics, one has to understand what post-modern philosophy considers to have been established. Kant, in his Critique of Pure Reason, showed the limits to the old Aquinas method of trying to ground philosophy in reason. There were two movements in reaction to this which sought an alternative method to ground philosophy. The analytic tradition attempted to base philosophy on mathematics, logic and the hard sciences. The Continental tradition attempted to base philosophy on perception, linguistics, and the modern sciences of philosophy and sociology.
Both movements failed. The analytics ran up against the Munchausen Trilemma, which showed that none of our beliefs are justifiable, the limits of empiricism, in that we cannot ever know what is true (therefore we can never have either justified or true beliefs), the inability to do analytics in any language (see Two Dogmas of Empiricism), the limitations of reason –based knowledge (Godel’s incompleteness theorem), and the plurality of logic (with multiple logics, any choice one uses to reason with – is unjustified).
The Continental movement produced two main sub-movements, phenomenology and Structuralism. Phenomenology failed when phenomenologists could not agree on basic observations between themselves. Structuralism sought to discover essential features of our universe by finding common structures in language, institutions, and other fields of study. Structuralism was initially very successful, until they turned their analytic methods on structuralism itself, and discovered that the structures they thought they were discovering in the world, were actually intrinsic assumptions within structuralist methodology. I.E. – the structuralism misadventure revealed NOT features of the world, But features of human thought: we can often only see what our assumptions project onto what we think of as data. This discovery was the birth of post-modernism.
Subsequent psychology has shown that we are highly subject to rationalizing something we wish were true (see Kahneman’s “Thinking Fast and Slow”, where system 2 often just rationalizes what System 1 decides). And sociology has shown that societies tend to reinforce the viewpoints of those that are powerful and thrive in that society (see the state capture by Robber Barons in the late 1800s).
So, what post modernists consider to be established, is that none of: reasoning, societal consensus, or even facts or methodologies to explore our world are “objective” or trustworthy. All can be rationalization tools for some hidden agenda.
How does all this apply to morality? All societies have oppressed and oppressor groups. And societies will, generally, create institutions to reinforce the oppression, and “moral” rationalizations to continue it. With facts, logics, and definitions of terms potentially subject to rationalizations, it is not actually possible to evaluate the claims of oppressed peoples vs. oppressor peoples, as the facts, definitions, and even logic are all suspect. Post-modernists therefore do not assert absolute subjectivity, but instead assert PLURALITY of moral POVs, with no clear method to resolving their disputes.
Post-modernists propose to accept the moral narrative of the oppressed rather than that of the oppressors, because this is harm minimizing. The oppressed may WANT to oppress in their turn, but as a practical matter, no societal overturn movement has the power to create the sort of harm that powerful oppressors have. Therefore taking the side of the disadvantaged is almost always morally more optimum.
Note this entire moral argument implicitly references the potential for an actual objective morality. But objective morality is unknown, and all our actual moral reference frames are subjective with intrinsic bias. Post Modernism asks people to act morally despite our uncertain world.
There are other and plausibly more valid critiques of post modernism. Post modernists tend to lean toward rationalism, rather than pragmatism. This leads to a neglect of the pragmatic defenses of reasoning, empiricism, and a peer dialog over “reasoning, societal consensus, facts and methodologies to explore our world” as highly useful, even if they are not always 100% true. Accept these concepts pragmatically, and then dialog and debate over moral claims becomes possible, rather than abandoning dialog and justification.
And the Post Modernist claim that the “oppressed” have no power and cannot therefore do any oppressing themselves – is simply false. Every individual has some power in a societal context, and can cause harm to others. No matter what degree of oppression they may also be subject to. This critique would reduce the often limitless endorsement that PoMos frequently extend to the formerly oppressed -- ignoring and dismissing the harms that the oppressed themselves create with their own hostilities.