Polyandrous arrangements do exist in some cultures. Usually its less about sex, and more about inheritance or population disparities.
With multiple brothers married to the same woman-in say Nepal or wherever polyandry exists as a legitimate form of marriage-its to keep land in the family. And if one brother dies, well he has another.
Problem with polyandry is that men are jealous, and liable to treat children not theirs badly(this has been documented in such cultures in the Amazon).
Polygyny is much more common, whereas polyandry while not a modern invention-is anthropologically speaking a relatively uncommon practice. For obvious reasons.
Then again the above bunch of degenerates strike me as more trying to do some sort of polyamorous arrangement, which is less common and even more prone to instability.
This Wikipedia article ought to give some basic understanding of the practice.
Polyandry - Wikipedia