I’ve seen a few articles over the years on this topic. In China, for example, women are getting more picky, desperate males are resorting to mail order brides. Economist mentioned a Vietnamese scam where the bride shows up, they marry in return for a large payment, then she disappears with the money. Possibly to rinse and repeat.
There was an article too about black women in America. Significant numbers of young black males are incarcerated, that black women looking for a black partner have slim pickings - aggravated by the fact that the early 20’s males are most likely to be in jail. Worse yet are the prospects for educated black professional women. One woman mentioned discovering that her boyfriend was cheating, only to be told “I didn’t know this was an exclusive arrangement.” Demand for free males is high enough that a free black male can have multiple relationships.
Another data point that might make sense is that males may have a propensity for younger females as partners - thus diminishing the pool of females for men in their 20’s; it’s not just millionaires or some presidents that snag women decades younger, and sometimes several in series.
A similar article blamed polygamy for ISIS (Daesh). If the top 5% of male middle eastern society can afford the quota of 4 wives, then the bottom 15% will never find a wife. The article suggested that horny 20-year-olds, not religious fanaticism, is what drove young men to join such a fanatical group. Note that one of the features of ISIS was that they simply kidnapped “eligible” females from the lands they conquered and awarded them to their fighters. Forget about 75 virgins in Paradise, these guys were in it for one live female on earth.