First, they have to prove their claim, which they can’t.
Don’t let them make YOU play defense. They make the claim that gender is a pure social construct, so make THEM play defense on the issue.
Second, we have strong evidence that gender is rooted in biology. In homosapiens, the male body is more suited for hunting and conflict, the female body less so. The more narrow hips of a male makes him more suitable for running. The lack of breasts make males more capable of throwing that rock or spear. The tendency for males to be larger and stronger than females solidifies their advantage over females in these activities.
On the flip side, the female body is uniquely adapted to caring for children and is dramatically more burdened by creating those children. In a time before both 1. baby formula and 2. medical science making pregnancies nearly 100% successful, this meant that having and caring for the next generation was a young female’s primary role.
Opened up across world culture throughout human history, we see roughly the same male dominance over virtually all activities outside the household. Exceptions where females were engaged in these sorts of activities are notable because they’re so rare relative to what is commonly seen across the species.
Gender is absolutely rooted in sex. But please be charitable to these confused folks. They do and should have the right to be weirdos if this is a genuinely free society. And you should have the right to freely and openly disagree with them.