Why are males and females 50%-50%?

[cthulu2016]

33. True, but that is not the mechanism

Any sex ratio is possible within the framework of an even number of X and Y sperm. In human the male sperm are much, much better at their job and manage to reach an ovum at a 170:100 ratio. But since a large number of male fetuses spontaneously abort, by birth the ratio is down to 105:100. And after birth down to 1:1.

In our case, since the X and Y production is fixed at 50-50, in order to acheive the evolutionarily optimal (or most stable) ratio of individuals (1:1) has involved selection of super-motile male sperm to overcome the high failure rate of male fetuses. The point being that the sex ratio is not an effect of the fact that the sperm types are 50-50 (and you're quite correct that they are 50-50 and that it would be hard to change)

That means that if we waved a magic wand and made the sperm ratio 75% female and 25% male evolution would, over time, handicap female sperm or enhance male sperm in such a way that we would still end up at a practical 50-50.

And if bears ate 90% of male children at age five we would, over time, see male birth outnumber female births 10:1. And so on.