Bottom line is.
If sex is a construct.
That means other mammals eg cats, elephants, chimps, lions, rabbits. Just. Guess?
No general sex specific behaviours?
Before early humans developed speech. Didn't notice any difference between the sexes?
With all this stuff a lot of good points applied to things randomly to be bamboozle.
So here.
Humans generally like to categorise. True.
Sometimes the categorisation turns out to be inadequate, incorrect. True.
Often it's because discovered significant differences within things grouped together. More complex than realised before. True.
Biases/ assumptions had meant missing stuff. True.
Now, it's all been turned around. The categories and what in them. Is irrelevant. A tree is a tree no matter what we call it, what category we put it in.
If it gets categorised as a fish, or stone, or a mammal.
We've got it wrong.
The tree is still a tree.
I think maybe your friend is on the popular argument.
That humans categorising sex as two options. Was an error because spectrum, clownfish etc.
Two sexes is an error that needs to be revised.
But. Whatever categories.
Mammals come in male and female. Penis vagina. Live young. Breast milk.
And all this confusing, pseudo scientific nonsense. Pushed out to confuse, divert, frustrate.
Never ever says why any of it means can access things that used to be single sex, IE penis/ vagina. Nothing else.