Polyandry (what you meant) is a woman with more than one husband, polygyny is a man with more than one wife.
Polyandry has virtually no historical precedent because the strongest man in such a relationship will simply kill his rivals and take the woman for himself - it's a biological reality in mammalian species. Men don't share. ;)
Polygyny was historically common because wars killed off a disproportionate number of young men. For a young woman facing economic hardship, being one of several wives to a wealthy and powerful man was not a bad deal.
Polyamory is in fact perfectly legal (see: Hefner, Hugh) - it's just that government-defined marriage and its attendant benefits can only legally be conferred on one other person at a time. And polyamory is widely practiced informally - there are plenty of women out there who would rather have 33% of Brad Pitt than 100% of Morty the Accountant. I don't think polygamy will be legalized any time soon, however, because it messes up government projections for things like Social Security in a way that gay marriage doesn't.