Monogamy has never been the main thing. However, with the equalizing of sexes in marriage according to the law, I don’t see how anything but monogamy can be legally until a lot of work is put into defining how three equal people can be married.
A form of polygamy is available to the upper class; it is called having a mistress. However, the mistress has no marriage rights; any rights would come from being the parent of a joint child.