Terisa and Matt and Vera and Larry-along with Scott, who's also at this dinner-are not swingers, per se; they aren't pursuing casual sex. Nor are they polygamists of the sort portrayed on HBO's Big Love; they aren't religious, and they don't have multiple wives. But they do believe in "ethical nonmonogamy," or engaging in loving, intimate relationships with more than one person-based upon the knowledge and consent of everyone involved. They are polyamorous, to use the term of art applied to multiple-partner families like theirs, and they wouldn't want to live any other way.

Terisa, 41, is at the center of this particular polyamorous cluster. A filmmaker and actress, she is well-spoken, slender and attractive, with dark, shoulder-length hair, porcelain skin-and a powerful need for attention.

Twelve years ago, she started dating Scott, a writer and classical-album merchant. A couple years later, Scott introduced her to Larry, a software developer at Microsoft, and the two quickly fell in love, with Scott's assent. The three have been living together for a decade now, but continue to date others casually on the side.

Recently, Terisa decided to add Matt, a London transplant to Seattle, to the mix. Matt's wife, Vera, was OK with that; soon, she was dating Terisa's husband, Larry. If Scott starts feeling neglected, he can call the woman he's been dating casually on the side. Everyone in this group is heterosexual, and they insist they never sleep with more than one person at a time.

It's enough to make any monogamist's head spin. But the traditionalists had better get used to it.

Researchers are just beginning to study the phenomenon, but the few who do estimate that openly polyamorous families in the United States number more than half a million, with thriving contingents in nearly every major city. Over the past year, books like Open, by journalist Jenny Block; Opening Up, by sex columnist Tristan Taormino; and an updated version of The Ethical Slut-widely considered the modern "poly" Bible-have helped publicize the concept.

Today there are poly blogs and podcasts, local get-togethers, and an online polyamory magazine called Loving More with 15,000 regular readers. Celebrities like actress Tilda Swinton and Carla Bruni, the first lady of France, have voiced support for nonmonogamy, while Greenan herself has become somewhat of an unofficial spokesperson, as the creator of a comic Web series about the practice-called "Family"-that's loosely based on her life. "There have always been some loud-mouthed ironclads talking about the labors of monogamy and multiple-partner relationships," says Ken Haslam, a retired anesthesiologist who curates a polyamory library at the Indiana University-based Kinsey Institute for Research in Sex, Gender and Reproduction. "But finally, with the Internet, the thing has really come about."

With polyamorists' higher profile has come some growing pains. The majority of them don't seem particularly interested in pressing a political agenda; the joke in the community is that the complexities of their relationships leave little time for activism. But they are beginning to show up on the radar screen of the religious right, some of whose leaders have publicly condemned polyamory as one of a host of deviant behaviors sure to become normalized if gay marriage wins federal sanction.

"This group is really rising up from the underground, emboldened by the success of the gay-marriage movement," says Glenn Stanton, the director of family studies for Focus on the Family, an evangelical Christian group. "And while there's part of me that says, 'Oh, my goodness, I don't think I could see them make grounds,' there's another part of me that says, 'Well, just watch them.' "

Conservatives are not alone in watching warily. Gay-marriage advocates have become leery of public association with the poly cause-lest it give their enemies ammunition. As Andrew Sullivan, the Atlantic columnist, wrote recently, "I believe that someone's sexual orientation is a deeper issue than the number of people they want to express that orientation with." In other words, polyamory is a choice; homosexuality is not. It's these dynamics that have made polyamory, as longtime poly advocate Anita Wagner puts it, "the political football in the culture war as it relates to same-sex marriage."

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."

The trio have had emotional moments. Scott had a hard time the first time he heard Larry called Terisa "sweetie" nine years ago. Larry was nervous when Terisa began semiseriously dating somebody outside the group. There are times when Scott has had to put up with hearing his girlfriend have sex with someone else in the home they share. And there have been moments when each of them have felt neglected in their own way. But they agreed early on that they weren't going to be sexually monogamous, and they are open about their affairs. "So it's not as if anybody is betraying anybody else's trust," says Larry.

There are, of course, some things that are personal. "Terisa doesn't tell me a lot of the private stuff between her and Matt, and I respect that," says Scott. When there are twinges of jealousy, they talk them out-by getting to the root of what's causing the feeling. "It's one of those things that sounds really basic, but I think a lot of people in conventional relationships don't take the time to actually tell their partner when they're feeling dissatisfied in some way," says Terisa. "And sometimes it's as simple as saying, 'Hey, Larry,' or 'Hey, Scott, I really want to have dinner alone with you tonight-I'm feeling neglected.' We really don't let anything go unsaid." As Haslam puts it: "It's all very straight forward if everybody is just honest about what's going on in their brains-and between their legs."

Larry and Terisa married last year-with Scott's permission-in part for tax purposes. Larry owns the house they all live in, and Scott pays rent. Household expenses require a complicated spreadsheet. Terisa, Larry, and Scott all have their own bedrooms, but sleeping arrangements must be discussed.

Larry snores, so Terisa spends most nights with Scott-which means she must be mindful of making up for lost time with Larry. Terisa and Larry only recently began dating Matt and Vera, after meeting on Facebook, and now every Friday, the couple bring their son over to the house and the three of them stay all weekend. Matt will usually sleep with Terisa, and Vera with Larry, or they'll switch it up, depending on how everyone feels.

The child, meanwhile, has his own room. And he's clearly the most delicate part of the equation. Matt and Vera have asked NEWSWEEK not to use their last names-or the name of their child-for fear, even in liberal Seattle, they might draw unwanted attention. Though Terisa doesn't have children-and doesn't want them-she adores Matt and Vera's son, who calls her Auntie.

Recently, the child asked his father who he loved more: Mommy or Terisa. "I said, 'Of course I love momma more,' because that's the answer he needed to hear," Matt says. He and Vera say they are honest with him, in an age-appropriate way. "We don't do anything any regular parents of a 6-year-old wouldn't do," he says. For the moment, it seems to be working. The child is happy, and there are two extra people to help him with his homework, or to pick him up or drop him off at school. They expect the questions to increase with age, but in the long run, "what's healthy for children is stability," says Fischer, the anthropologist.

It's a new paradigm, certainly-and it does break some rules. "Polyamory scares people-it shakes up their world view," says Allena Gabosch, the director of the Seattle-based Center for Sex Positive Culture. But perhaps the practice is more natural than we think: a response to the challenges of monogamous relationships, whose shortcomings-in a culture where divorce has become a commonplace-are clear.

Everyone in a relationship wrestles at some point with an eternal question: can one person really satisfy every need? Polyamorists think the answer is obvious-and that it's only a matter of time before the monogamous world sees there's more than one way to live and love. "The people I feel sorry for are the ones who don't ever realize they have any other choices beyond the traditional options society presents," says Scott. "To look at an option like polyamory and say 'That's not for me' is fine. To look at it and not realize you can choose it is just sad."

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