‘Sex is a social construct’? 
What, they genuinely think someone just made up the concept of men and women being different? 
Has anyone ever told them how babies are made? Told them how girls experience sexual harassment before they’ve even got through puberty? How women have different and often unmet physical needs around child birth, periods and menopause? How women are killed by their male partners (every week in the UK)?
Those just a few of the sex-based problems which girls and women bear the burden of.
Only a very over-privileged, sheltered, unempathetic and craven man-pleaser would ever try to claim that physical sex is made up.
Look around the world for one second and you can see what deliberate and staggering misogyny is behind anyone making that claim.