There is a secluded tribe of people in the Yunan province in southern China that don't get jealous when their best friends sleep with the hot guy they've been crushing on for about a year and a half. That's because he's already slept with her at least once, and, along with the rest of the male population, he sleeps with whom ever will have him. I know I know, this sounds like your ex boyfriend doesn't it. But seriously, this is an anti-monogamist society that functions. Almost every male in the population knows where the women's bedrooms are and the women have all the power to deny them, but they don't. Women live with their brothers and their mothers together to raise the children they have with the many men they sleep with. Fathers and Husbands aren't necessary. But there are a few taboos: Women and men in the same household do NOT sleep together. When going to the movies they won't even sit in the same row, should a scene display an emotional or physical connection between a man and a woman. These "visits" are kept completely private. Men will throw pebbles at the window of the woman he desires and if she desires him she will let him in. If two should meet and desire to meet later, they will secretly exchange items, or even glances. The women do not visit the men. If she has no visitors, she is left... a little frustrated. Women do decline offers, especially to avoid a double booking, although she may have more than one visitor a night.
Before you buy your plane tickets, I'll warn you that it's not all sunshine and roses. There is also the chance that a father and daughter may sleep together, since everyone is considered "fatherless." Also over half the male population has syphilis, many women are steril, more and more people are born deformed, and the population is stagnating.
Yep, I love cultural anthropology.