Fewer Male Ancestors

Something about how only one in two males reproduce, but on average every human female has offspring?

This kind of guy has no no problem passing genes on. There are many more single mothers than single fathers for this reason.

I think it just shows that it is women who decide who reproduces. It is probably easy to find some sperms somewhere (these days there are also sperm banks), but it is hard to find a willing womb. Hence only the top fraction of men gets to reproduce (with several women), because women can just stick to the top fraction of men for reproduction.

You are missing the point here, may be. Man could not be reproductively successful without women. If a boy has thousands of children, there needs to be corresponding women who can carry them.

Evidence that some couples can only have one sex.

While a son would have a much lower chance of reproducing, if he does, he'll reproduce a lot. I think that's the balance.

Not really, everyone (or just every woman, or man, however) could be predisposed to whether they give birth to sons or daughters; on average then it'd still work out OK. (Might even be better, since it'd discourage interbreeding.)

mating has historically almost always been a pretty hostile and unfair experience. That is why female ancestors outnumber male 2:1.

Probably something like the existence of only two sexes.

See Bateman's principle [1]. In most species, variability in reproductive success is greater in males than in females. This is also true for humans (even today) [2], modern human may have twice as many female ancestors as male ancestors.This is fairly intuitive result when you consider gestation time - imagine a scenario where there are more male than female in a given moment for a given species. At most each female got fertilized once, so the number of distinct male parents is less than