Science

The Mosuo (a Himalayan Chinese ethnic group) and the Himba (the Namibia and Angola border region) are both very interesting in this respect.

The Mosuo do not have marriage (although their now extinct aristocracy and nearly extinct priesthood did/do). They live in their family home all their lives. Mosuo women have sexual freedom; no one cares how many partners they have or whether there's more than one relationship ongoing simultaneously, and it's not always certain who someone's father is (the matriarchs keep tabs on relationships to help ward off accidental incest). Fathers are not expected to contribute; they raise the children of their sisters and contribute financially to their birth-family household. But, despite the freedom to have many partners on the go with no adverse social consequences at all, most Mosuo tend towards serial monogamy, with some relationships lasting years, others for a lifetime. Those that move to urban China for work tend to adopt traditional marriage because the Mosuo lifestyle is not practical without a whole household to help care for the children.

The Himba, on the other hand, do have marriage and, polyamory for men who are wealthy enough to support more than one wife. But wives are free to have as many other partners as they wish and many women, married or unmarried, will have several on the go at the same time. If a woman wants to sleep with a married man, she gets permission from his wife. Women will often have children before they get married and those they have after marriage are not necessarily fathered by their husbands. It's not an issue because they're herders; children are a source of wealth (more goats can be herded) so the men do not care who the fathers of their children are.

I'm no anthropologist, and I hope I've done these groups justice with my brief descriptions. It's a fascinating topic, especially with respect to polyamory in a rich world context.