Perhaps I don’t understand China’s history well enough. Has large-scale polygamy really been that prevalent for a lot of China’s history? I didn’t realize that.
Monogamy is a Western export. Outside of the West, polygamy was the rule prior to the wholesale introduction of Western legal codes (believed by non-Western modernizers at the time to be the key to the material success and military power of the West) in non-Western countries. Men had as many wives as they could afford, whereas those who couldn't afford wives either rented or did without. There's reason to think that even in the monogamous UK, a fair number of men went without - John Derbyshire at the National Review mentioned way back that the vast majority of the UK's native population is related by DNA to some branch of the aristocracy. It appears that the lower classes did not pass on their DNA, whereas a significant chunk of the more remote branches of the aristocracy descended* into the middle and lower classes.
* Incidentally, my nom de plume, "Zhang Fei", was a warlord who worked for a distant relative of the Han emperor, Liu Bei, who attempted to revive the Han Dynasty (2nd century BC to 2nd century AD) after its collapse. The occupation of this very minor aristocrat? He made straw (i.e. dried grass) sandals for a living. His exploits - real or legendary - were detailed in the 14th century Chinese novel, the Romance of the Three Kingdoms.