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In terms of "sex" I agree with that statement.

Well, "social construct" is a meaningless term.

How so?

It has no application in reality.

I'm not sure I would agree with that. Humans use language to create meaning within their reality. Again, biological sex is a thing but gender is just an idea, a concept. Gender has largely remained a binary concept because it has never been questioned by society. However, recently individuals and groups have begun to question the current paradigm and the status quo and as a result, the concept of gender is changing, shifting and evolving. This is only one example of how something can be a social construct, so I personally think that there are plenty of applications for it within reality.