Only You. And You. And You.

Polys themselves are not visibly crusading for their civil rights. But there is one policy issue rousing concern: legal precedents concerning their ability to parent. Custody battles among poly parents are not uncommon; the most public of them was a 1999 case in which a 22-year-old Tennessee woman lost rights to parent her daughter after outing herself on an MTV documentary. Anecdotally, research shows that children can do well in poly families—as long as they're in a stable home with loving parents, says Elisabeth Sheff, a sociologist at Georgia State University, who is conducting the first large-scale study of children of poly parents, which has been ongoing for a decade. But because academia is only beginning to study the phenomenon—Sheff's study is too recent to have drawn conclusions about the children's well-being over time—there is little data to support that notion in court. Today, the nonprofit Polyamory Society posts a warning to parents on its Web site: If your PolyFamily has children, please do not put your children and family at risk by coming out to the public or by being interviewed [by] the press!

The notion of multiple-partner relationships is as old as the human race itself. But polyamorists trace the foundation of their movement to the utopian Oneida commune of upstate New York, founded in 1848 by Yale theologian John Humphrey Noyes. Noyes believed in a kind of communalism he hoped would fix relations between men and women; both genders had equal voice in community governance, and every man was considered to be married to every woman. But it wasn't until the late-1960s and 1970s "free love" movement that polyamory truly came into vogue; when books like Open Marriage topped best-seller lists and groups like the North American Swingers Club began experimenting with the concept. The term "polyamory," coined in the 1990s, popped up in both the Merriam-Webster and Oxford English dictionaries in 2006.

Polyamory might sound like heaven to some: a variety of partners, adding spice and a respite from the familiarity and boredom that's doomed many a traditional couple. But humans are hard-wired to be jealous, and though it may be possible to overcome it, polyamorous couples are "fighting Mother Nature" when they try, says biological anthropologist Helen Fisher, a professor at Rutgers University who has long studied the chemistry of love. Polys say they aren't so much denying their biological instincts as insisting they can work around them—through open communication, patience, and honesty. Polys call this process "compersion"—or learning to find personal fulfillment in the emotional and sexual satisfaction of your partner, even if you're not the one doing the satisfying. "It's about making sure that everybody's needs are met, including your own," says Terisa. "And that's not always easy, but it's part of the fun."

It's complicated, to say the least: tending to the needs of multiple partners, figuring out what to tell the kids, making sure that nobody's feelings are hurt. "I like to call it polyagony," jokes Haslam, the Kinsey researcher, who is himself polyamorous. "It works for some perfectly, and for others it's a f--king disaster."

Some polyamorists are married with multiple love interests, while others practice informal group marriage. Some have group sex—and many are bisexual—while those like Greenan have a series of heterosexual, one-on-one relationships. Still others don't identify as poly but live a recognizably poly lifestyle. Terisa describes her particular cluster as a "triad," for the number of people involved, and a "vee" for its organization, with Terisa at the center (the point of the V) and her two primary partners, Scott and Larry (who are not intimate with each other) as the tips of each arm. Other poly vocabulary exists, too: "spice" is the plural of "spouse"; "polygeometry" is how a polyamorous group describes their connections; "polyfidelitous" refers to folks who don't date outside their menage; and a "quad" is a four-member poly group.

It's easy to dismiss polyamory as a kind of frat-house fantasy gone wild. But in truth, the community has a decidedly feminist bent: women have been central to its creation, and "gender equality" is a publicly recognized tenet of the practice. Terisa herself is proof of that proposition, as the center of her cluster. She, Scott, and Larry have all been polyamorous since meeting in the Bay Area in the '90s, where they were all involved with the same theater community.

Terisa and Scott started dating first. Both were getting out of long-term monogamous relationships—Terisa had been married for six years—and knew they wanted something different. They fell in love, and though they were committed, they began dating around. Two years in, Scott introduced her to Larry, a pit violinist and mutual acquaintance. When Larry was offered the Microsoft job in Seattle, he asked Terisa and Scott to go with him. "We were like, 'Wow, are we really going to do this?' " Terisa remembers. "And we sort of just said, 'Well let's jump in!' "

It wasn't long before they realized there was a thriving community of Seattleites living the same way. There were local outings, monthly poly potlucks, and a Sea-Poly e-mail list that served to keep everyone informed. Larry even found a poly club for Microsoft employees—listed openly on the company's internal Web site. (Microsoft declined to comment on the message board, or whether it still exists.) The trio has been together ever since, and they share a lakeside home in Seattle's Mt. Baker neighborhood, where they have a vegetable garden and three dogs. They often go on walks along the lake, hand in hand in hand. "I think if we were all given a choice, everyone would choose some form of open relationship," Scott explains, sitting in the family's hillside gazebo overlooking Lake Washington. "And I just like variety," Terisa chimes in, laughing. "I get bored!"