Probably cause when we were kids is when we had our first crushes, so maybe stems from that.

Because the cultures refuse to acknowledge equality and just say sexual friends.

It's easier to control by dividing into two sexes.

Yet too many divisions it becomes harder to control, the more divisions there are.

This polarity (opposites) is used in everything, so it can be easily controlled. It's either good or bad. And nothing in-between.

Yet there real is nothing in reality, that is a total opposite.

It's all about control.

It’s called many “something else” names as well.

probably heavily puritanical and fits well as an euphemism