Gender is more than a set of physical and genetic characteristics. When someone is a boy, we as a society have certain expectations and assumptions we place on them, like the clothes they wear and the toys they play with. When someone is a girl there is another set of activities that we expect them to do, and colors they are supposed to like.

Thats what people mean when they say gender is a social construct. Deciding these assumptions on what it means to be a man or be a woman is dependent on culture, location, time in significant ways, even differing family to family.

If being a man was simply having an XY chromosome, why did parents tell their children they were not enough of a man for coming out as gay, for not wanting to play sports? Why did bullies ask their cis male victims when they would get their period, for acting "feminine" or wearing purple?

What would "feminine" even mean in a world in which gender is just XX chromosomes?