Why are males and females 50%-50%?

[HereSince1628]

22. It seems unlikely to me that a bottle-neck would lead to a change in species mating-systems

Bottle-necks result in reductions of genetic variability. Mating systems within a species are highly conserved and there probably isn't enough genetic variability in the controlling factors for mating systems to result in a shift when genetic variability is reduced. Two species that we know have gone through bottle-necks, elephant seals and cheetahs, have pretty much exactly the same mating systems as their near relatives that haven't experienced known bottle-necks.

I'd suggest that mating systems be considered solutions to past fitness tests

The genetics that make mating systems heritable may be assumed to always subject to mutation and selection, but the variations are also some of the most conserved heritable features of a species.

Mating systems (monogamous vs polygamous) emerge as the myriad simultaneous fitness problems are solved in the evolutionary history of a species. What is seen today is the current end-product of the 'ghost' of past fitness tests. Those fitness tests act on mating process, embryonic processes and post-embryonic developmental and maturation processes.

Selection pressure doesn't necessarily begin or end with number of sexual partners with which individuals successfully copulate.

The developmental condition of the offspring (altricial-relatively helpless at birth vs precocial-relatively independent at birth) places signigicant burderns on parent(s). Selection for survival of the offspring is not just on the 'infants' it is also on the adults via patterns of paternal care.
This characteristic developmental state of the newborn is both a solution to survival problems and a biotic platform that is acted upon by selection, for atricial offspring such as human babies, it can only work with the concomitant availability of 'workable' phenotypes produced by 'workable' genetic variation in the parent(s).

Parental care may include provisioning with food, providing protection, transportation or shelter. And particularly in humans, it includes the endowment of offspring with culture that promotes in broad ways to the fittness of the offspring and the perpetuation of the genetic lineage of both parent and offspring. The demands of parental care have many requirements/costs.

Mating systems represent workable solutions to the fitness costs and fitness benefits to producing a successful generation of offspring.

Monogamous mating systems, and variations on parental care that can include both-sexes cooperation in parenting are also representation of workable, heritable solutions.