Having read many, many books on Tibet and Tibetan buddhism, I can assure you that both polyandry and polygamy was practised in Tibet, polyandry more by the poor, and polygamy more by the rich, and that many commentators viewed tibetan society at least in part to be matriarchal. in that the women were in charge of the homes, businesses and farms and made most of the decisions as well as doing much of the work, largely in part because a huge percent of the men were away in the monasteries. One of the last true matriarchal cultures still exists on the borders of Tibet, the Mosuo people.
heres a reference to multiple wives among the nobility, admittadly appears to be a chinese source, but accurate in this respect; http://www.kalachakra.org/articles/marriage.shtml
18 years ago I made one of the most important decisions of my life and entered a local Cambodian Buddhist Temple as a temple boy and, for only 3 weeks, an actual Therevada Buddhist monk. I am not a scholar, great meditator, or authority on Buddhism, but Buddhism is something I love from the Bottom of my heart. It has taught me sobriety, morality, peace, and very importantly that my suffering is optional, and doesn't have to run my life. I hope to give back what little I can to the Buddhist community that has so generously given me so much, sincerely former monk John