"Polygamy" is a word that really isn't used to mean the thing its component parts imply (marriage for polyamorous people.)
Instead, it's usually both mixed-age and arranged: effectively, families coercing their daughters to join the harem of a rich man. (There may also be a dowry, in which case the family is selling their daughter into the harem of a rich man.)
This is a pretty bad thing, but it's hard to write a fully-general (e.g. gender-neutral) legal definition of it that doesn't also cause problems for regular poly people.
Instead, it's usually both mixed-age and arranged: effectively, families coercing their daughters to join the harem of a rich man. (There may also be a dowry, in which case the family is selling their daughter into the harem of a rich man.)
This is a pretty bad thing, but it's hard to write a fully-general (e.g. gender-neutral) legal definition of it that doesn't also cause problems for regular poly people.