Polyandry is virtually unknown in human history because men are aggressively territorial and have a tendency to violently dispose of other men who sleep with their women, regardless of what social arragnements are in place.
Polygyny is, on the other hand, common in human history - largely for economic reasons. Women could assure a far better life for themselves and their children being one of a wealthy and powerful man's five wives than by having a stableboy all to themselves. ;)
I can hear the NOW hags already ... "But that's not fair! We have equal rights, don't we? You want to oppress us!"