Why are males and females 50%-50%?

[cthulu2016]

18. Exactly (I was trying to say that in the OP but may have failed)

An advantage on either side is exploited until it becomes a disadvantage, oscillating around a stable point.

So these Chinese folks opting for boys may benefit due to the inertia of existing culture, but it cannot last.

Parents chose a boy because the boy can contribute to their security in retirement. I predict that as women become scarce they will become a valuable commodity and parents will charge potential suitors the same money for their daughter's hand they would have gotten from a son. A reverse dowary.

No matter what happens, as long as women want husbands and men want wives there's only so much room for discrepency before the society shifts.

I am fascinated by the social mores of Australia that persist from it's early days of a society with very few women. Or the social mores of post WWI Europe where there was a big shortage of men. Our societies are calibrated to roughly 50-50 and any substansial deviation from that breaks down sexual custom quite quickly, since our sexual customs are both derived from and supportive of that expected mix.