At least in the US and other Western countries, where there is little sex-selection at or before birth, the overall ratio of men to women has been remarkably stable. What has changed in the past 40 years is the ratio of men to women among the relatively well-educated. Men and women engage in assortive mating, each attracted to the other within fairly narrow race, income, and education categories. There are more high-earning college-educated women than high-earning, college-educated men among 25-44 year olds now, so competition among women for highly “marriageable†men has increased. This is driving many trends we observe, such as late marriages, the increase in never marrieds, and many changes in sexual behavior.
I want to recall that this has been occurring in Russia(Soviet union) as well. The males are much less motivated (drink and party) while the women are leading in education and income.
Yes, the ratio at birth is fairly even but men go to war and have accidents. And a big percentage of them are actively and publicly gay. The percentage of straight college educated men is less than the college educated women.
Then you have the fact that women tend to marry up, e.g., taller, better educated, more affluent, higher status, older and you have a problem for many women. Female college basketball players have a small pool to pick from.
Then there’s the Bell Curve