Science

Yes, sex is a social construct in terms of that it's just a thing that is assigned and a label that has been assigned to people based on what it's assumed their bodily makeup.

Doctors and others don't exactly do karyotype or other genetic testing at birth, they look at the genitals and usually apply one of two boxes to you and unless you look into it that label follows you around the rest of your life. Often, if you don't fit into those boxes they try to make you.

It isn't helpful nor accurate, because bodies are not the same. This is what we mean when we say sex is a social construct, it's just a label, a shortcut through language meant to imply homogeneity forgoing accuracy and meaning, and it doesn't mean anything because it's not specific enough to.

Sex is a social construct in all the ways gender is, because as stated it's just a label that gets assigned to you. What with HRT, surgery etc most, if not all biological characteristics of a body can be changed and this makes the distinction useless. Especially because people are so hung up on what bodies look like and what their functions are supposed to be, yes this is partially to do with gender but it also speaks to 'sex' too.

Even scientists understand this now and though I don't necessarily agree nor disagree with this, they call sex a bimodal distribution, not a distinct binary any more.

But regardless of what it's called or how many 'sexes' are recognised, it's still a social construct because it's taking a bunch of characteristics and applying labels to it. That's it.