I think they’re being glib, but sex absolutely is a social construct. What exactly constitutes THE properties of the female and male sex is defined by society more than biology. For example, people who have androgen insensitivity syndrome develop perfectly normal looking “female” bodies, but their genetic sex is “male”. People generally still consider those individuals to be “female”. If you want to get down to it, all words are socially defined. They usually attempt to describe or capture reoccurring details about the observable world, but nature (especially biology) has too much variety and weird exceptions for any simple linguistic model to perfectly match the physical world. That’s doubly true for a linguistic model used in everyday life as opposed to scientific discourse.

That’s not to say that “sex isn’t real” or some absurd position like that. There is a real phenomenon in biology where genetic and phenotypical variance tends to follow a limited set of patterns. Biologists call those patterns “sex”. But, the idea that “sex” as we use it in normal day-to-day life is a rigidly defined system based on hard biological facts is laughably false. When was the last time you insisted on seeing someone’s genetic karyotype test results? When was the last time you asked someone to drop their pants so you could examine their genitalia and determine their sex?

The phrase “sex is a social construct” is technically true in the same way that “chairs are a social construct” is true. The word describes a fuzzy subset of the set of all things, and what we consider in or out of the set is largely based on what we think should belong to it, and that is absolutely influenced by societal expectations. Where do chairs end and tables begin? Where does the female sex end?

Once you get to that point, the phrase almost has no meaning. If all word definitions are social constructs, then that’s table stakes and it doesn’t make sense to call out a specific word as having that property. Surely the phrase “is a social construct” must mean something more than “is like any other word”. Indeed, the meaning of that phrase is larger than what it literally says. There is a socially defined meaning that is attached to it. That phrase implies the word in question “is arbitrarily defined (more than most words)” and “is up for interpretation (more than most words)”.

If you agree with me this far, then the right way to frame this discussion is “should we pin ‘sex’ as it is used in every day life to specific physical properties or should we allow it to be more porous?”. This isn’t as straightforward as it might seem. If we pin it, for example, to genetic karyotype, then we need to stop using it in contexts where the genetics of the person are unknown and must admit the existence of several additional sexes (e.g. X, XXY) AND recognize it’s no longer useful for describing the phenomenon of what a person looks like or how their body works. If we make it porous, then we will endlessly debate about whether boundary cases fall into one bucket or the other and possibly hurt some feelings along the way.

The fun thing is that we get to pick what words mean, and we get to pick what we do with “sex”. Language is, after all, a social construct.