I feel like I've missed something fundamental here.
I get the idea in principle: that we create categories & labels to organise our world, & these evolve to accommodate our developing understanding of the things they describe.
But to argue that this means sex is a social construct is surely to reduce indiscriminately all scientific definitions and the accumulated knowledge of ages to the same. All meaning's ultimately "constructed", after all.
So, by this logic, isn't a whale a social construct? And Omicron must be. And a Dairy Milk bar (is it still Dairy Milk since they changed the shape?! - social construct!)
For this argument to work, then, surely you actually have the change the definition of "social construct" itself, to embrace a far wider range of human knowledge than is conventionally meant by it. And, in so doing, you degrade ITS meaning, too: whereas, previously, "social construct" was an implicit acknowledgement of the fallibility and transitoriness of socially constructed VALUES, in applying it to science, you reduce ALL knowledge to a reflection of such values, discounting lived experience, research and evidence.
And from that perspective, it IS a privileged, elitist approach. Your friend, Foucault, Judith Butler etc., can afford to wax lyrical in this way. Most of the world's women can't.
Fundamentally, this denial of sex rests on an epistemological argument - how do we know what we know - in line with post-modern queer theory etc. that may feel thrillingly intellectual, but is, itself, a social construct - the product of a post-truth society so complacent that it's starting to attack the very foundations it rests on.
(Not to hyperbolise...!)
Or HAVE I missed something?