To some degree, I agree with that. My agreement stems from Enlightenment roots though (also known as liberal or liberal democratic roots though), when 'God' was overturned and 'man' (with 'his' reason, etc.) became centred.
To put that another way, we (or at least the vast majority of human societies now and throughout history) have chosen to divide the human race into a binary based on particular characteristics (originally, genitals).
We could have chosen any other dividing characteristic (over 5'4 you are a tall human, and, under this, you are short, for example). To sidetrack myself consider poor old Pluto - is Pluto a planet or not? It depends, based on human defined criteria. There is nothing intrinsic about Pluto to say that it is or is not a planet. The power of defining is human based.
But, in relation to humans, we didn't categorise on the basis of size because it made sense (rationality again) to call one set of people, based on observable characteristics, Female and the other set Male and it didn't make sense (Goodies Apartheight aside) to divide according to height (let's leave 'race' aside here, that's another matter).
Modernist Enlightenment thought, to some degree, also recognises that how we name things is also about the relative power of different groups of humans. So Mary Wollstonecraft and our other early forebears called men to account for their power in claiming that only men possessed rationality and were deserving of rights and they pointed out that we could understand women as possessive of these too. There was some quite robust debate about this and it was an open debate because Enlightenment thinkers are wary of book-burning (and probably more so than most other traditions).
Being rational, and being concerned about other fundamental principles such as justice, equality, etc., enlightenment thinkers would also ask questions such as 'how does our choice to name women and men, or black skinned people and white skinned people tally with equality and justice (etc.)? In other words, if a human exercises their power to name something, who wins and who loses and on what bases do we make these kinds of decisions.
I first read postmodern texts in the 80s and I got a cold chill then because I could not find one that really offered up a convincing theory of power and how this works and how it might be sexed or largely in the hands of the intelligentsia, etc.
What terrifies me about a lot of the TRAs (who seem to borrow surface ideas from both liberalism and postmodernism, mixed with libertarianism and anarchism) is that they don't think through and are not willing to have debate about who has the power to name and interpret and wins and loses from their conviction that the 'truth' that sexed identity is a matter of personal interpretation (and thinking about it, this is as much of a truth as 'there are two sexes and some unfortunate individuals with chromosomal disorders' is, despite their pomo claims).
TL;DR - Postmodernism worries me because it lacks a theory of and recognition of (sexed) power.