And this is where it becomes difficult. But, the question must be posed, wrong in virtue of what. They are not absolute. They are, in effect, prescriptive to the extent that we believe they are. Here we can see the driving force behind much of postmodern political work.
Postmodernism is the post-marxist incarnation of the leftist emancipatory urge.
Postmodernists see various minority groups in society being marginalised because their beliefs, practices and identities are not acceptable to the broader public, and they set out to change the social values of society so that these groups can be accepted, so that they not be discriminated against because of their otherness, their difference. That is fine as far as it goes. However, the postmodern belief that all values are socially constructed leaves no answer to the question of why it is that the social values they are trying to construct in the wider society are better than the ones that already exist there. I shall furnish you with a trivial example in order to keep this as non political as possible. The existing social norm is that owning goldfish is bad form.