The key operative precept of post-modernism is that all reality is a social construct. Since no institution or normative behavior stems from natural cause, and there is no objective, external reality, then all institutions and attitudes are just social constructs. They exist only through the agreement of the participants.
Nothing can be sacred, since sanctification comes from outside, by definition.
If nothing is sacred, and institutions have no more reality than a children's amorphous game of ball, they deduce that any construct can be reconstructed through willful choice.
Even if you accept the precept that there is no objective, external (let alone universal) value system, you can still see the fundamental fallacy in this thinking.
Anyone who has ever tried to bring change into a hidebound organization knows that social constructs are far harder to change than any physical or legal structure. You can reorganize units, bring locations together, shuffle management, or get rid of half of the people. Still, underlying social organization will re-emerge as long as there is any vestige of continuity.
Much of the heat energy in the ongoing culture war arises from this inertia. Those who are so tiresomely labelled as "liberal", "progressive", the "Left", the "Cultural Elite", etc. represent a large force of people aimed at deliberately reconstructing every institution in Western life. They have decided, based on their own feelings, bereft of natural or religious law, that any institution observed by men for more than one hundred years cannot be endured. They are organized around the post-modern paradigm--armed with Hayakawa and Chomsky--and don't accept that some hidebound Neanderthals will not welcome forceful re-education.
I suppose that I follow a third way. I can agree that our institutions are social cosntructs. That does not mean that they can, or should be, tampered with lightly. The concept of "natural law" teaches that certain modes of behavior, certain morals, generate a more successful society. Our social institutions--like marriage--have undergone the same forces of competitive pressures and differential reproduction that drive neo-Darwinian evolution. That means the institutions we observe today--such as preserving the integrity of personal property--are the ones that worked.
There is an argument to be made that I'm advocating cultural imperialism. It could perhaps be seen that way, though such is not my intent. Rather, just as we should justifiably be wary of changing our own genetic code, we should be wary of making large changes to our social institutions. We do not know what will result. There are many paths down the mountain, but only one upward. Most random mutations result in death. Even well-planned changes have unintended, sometimes catastrophic, effects.