[dragonwriter]

> That only works in a polygamous society.

Most societies are polygamous when you look in terms of sex and reproduction, even if they aren’t when it comes to socially recognized family bonding.

> In monogamous societies like the UK the deaths of many men in the world wars led to "marriage gaps" where a roughly equivalent number of women were unable to marry and have children.

Unable to marry, maybe, unable to have children, less so, hence the spike in unmarried share of births around the end of WWII (probably even moreso in WWI, too, but the information I have starts in the 1920s.)