John Springer Collection-Corbis. CLICK FOR A PHOTO HISTORY OF MULTI-PARTNER RELATIONSHIPS.

Terisa Greenan and her boyfriend, Matt, areenjoying a rare day of Seattle sun, sharing a beet carpaccio on thepatio of a local restaurant. Matt holds Terisa's hand, as his 6-year-oldson squeezes in between the couple to give Terisa a kiss. His mother,Vera, looks over and smiles; she's there with her boyfriend, Larry.Suddenly it starts to rain, and the group must move inside. In theprocess, they rearrange themselves: Matt's hand touches Vera's leg.Terisa gives Larry a kiss. The child, seemingly unconcerned, puts hisarms around his mother and digs into his meal. 

Terisa and Matt and Vera and Larry—along with Scott, who's also atthis 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 dobelieve in "ethical nonmonogamy," or engaging in loving, intimaterelationships with more than one person—based upon the knowledge andconsent of everyone involved. They are polyamorous, to use the term ofart applied to multiple-partner families like theirs, and they wouldn'twant to live any other way.

Terisa, 41, is at the center of this particularpolyamorous cluster. A filmmaker and actress, she is well-spoken,slender and attractive, with dark, shoulder-length hair, porcelainskin—and a powerful need for attention. Twelve years ago, she starteddating Scott, a writer and classical-album merchant. A couple yearslater, Scott introduced her to Larry, a software developer at Microsoft,and the two quickly fell in love, with Scott's assent. The three havebeen living together for a decade now, but continue to date otherscasually on the side. Recently, Terisa decided to add Matt, a Londontransplant to Seattle, to the mix. Matt's wife, Vera, was OK with that;soon, she was dating Terisa's husband, Larry. If Scott starts feelingneglected, he can call the woman he's been dating casually on the side.Everyone in this group is heterosexual, and they insist they never sleepwith more than one person at a time.

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

Researchers are just beginning to study thephenomenon, but the few who do estimate that openly polyamorous familiesin the United States number more than half a million, with thrivingcontingents 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—widelyconsidered the modern "poly" Bible—have helped publicize the concept.Today there are poly blogs and podcasts, local get-togethers, and anonline polyamory magazine called Loving More with 15,000 regularreaders. Celebrities like actress Tilda Swinton and Carla Bruni, thefirst lady of France, have voiced support for nonmonogamy, while Greenanherself has become somewhat of an unofficial spokesperson, as thecreator of a comic Web series about the practice—called "Family"—that'sloosely based on her life. "There have always been some loud-mouthedironclads talking about the labors of monogamy and multiple-partnerrelationships," says Ken Haslam, a retired anesthesiologist who curates apolyamory library at the Indiana University-based Kinsey Institute forResearch in Sex, Gender and Reproduction. "But finally, with theInternet, the thing has really come about."

With polyamorists' higher profile has come somegrowing pains. The majority of them don't seem particularly interestedin pressing a political agenda; the joke in the community is that thecomplexities of their relationships leave little time for activism. Butthey are beginning to show up on the radar screen of the religiousright, some of whose leaders have publicly condemned polyamory as one ofa host of deviant behaviors sure to become normalized if gay marriagewins federal sanction. "This group is really rising up from theunderground, emboldened by the success of the gay-marriage movement,"says Glenn Stanton, the director of family studies for Focus on theFamily, an evangelical Christian group. "And while there's part of methat says, 'Oh, my goodness, I don't think I could see them makegrounds,' 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 thepoly cause—lest it give their enemies ammunition. As Andrew Sullivan,the Atlantic columnist, wrote recently, "I believe that someone'ssexual orientation is a deeper issue than the number of people theywant to express that orientation with." In other words, polyamory is achoice; homosexuality is not. It's these dynamics that have madepolyamory, as longtime poly advocate Anita Wagner puts it, "thepolitical football in the culture war as it relates to same-sexmarriage."

Polys themselves are not visibly crusading for their civil rights.But there is one policy issue rousing concern: legal precedentsconcerning their ability to parent. Custody battles among poly parentsare not uncommon; the most public of them was a 1999 case in which a22-year-old Tennessee woman lost rights to parent her daughter afterouting herself on an MTV documentary. Anecdotally, research shows thatchildren can do well in poly families—as long as they're in a stablehome with loving parents, says Elisabeth Sheff, a sociologist at GeorgiaState University, who is conducting the first large-scale study ofchildren of poly parents, which has been ongoing for a decade. Butbecause academia is only beginning to study the phenomenon—Sheff's studyis too recent to have drawn conclusions about the children's well-beingover time—there is little data to support that notion in court. Today,the nonprofit Polyamory Society posts a warning to parents on its Website: If your PolyFamily has children, please do not put yourchildren and family at risk by coming out to the public or by beinginterviewed [by] the press!

The notion of multiple-partner relationships is asold as the human race itself. But polyamorists trace the foundation oftheir movement to the utopian Oneida commune of upstate New York,founded in 1848 by Yale theologian John Humphrey Noyes. Noyes believedin a kind of communalism he hoped would fix relations between men andwomen; both genders had equal voice in community governance, and everyman was considered to be married to every woman. But it wasn't until thelate-1960s and 1970s "free love" movement that polyamory truly cameinto vogue; when books like Open Marriage topped best-sellerlists and groups like the North American Swingers Club beganexperimenting with the concept. The term "polyamory," coined in the1990s, popped up in both the Merriam-Webster and Oxford Englishdictionaries in 2006.

Polyamory might sound like heaven to some: a variety of partners,adding spice and a respite from the familiarity and boredom that'sdoomed many a traditional couple. But humans are hard-wired to bejealous, and though it may be possible to overcome it, polyamorouscouples are "fighting Mother Nature" when they try, says biologicalanthropologist Helen Fisher, a professor at Rutgers University who haslong studied the chemistry of love. Polys say they aren't so muchdenying their biological instincts as insisting they can work aroundthem—through open communication, patience, and honesty. Polys call thisprocess "compersion"—or learning to find personal fulfillment in theemotional and sexual satisfaction of your partner, even if you're notthe 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 multiplepartners, figuring out what to tell the kids, making sure that nobody'sfeelings are hurt. "I like to call it polyagony," jokes Haslam,the Kinsey researcher, who is himself polyamorous. "It works for someperfectly, and for others it's a f--king disaster."

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

It's easy to dismiss polyamory as a kind of frat-house fantasy gonewild. But in truth, the community has a decidedly feminist bent: womenhave been central to its creation, and "gender equality" is a publiclyrecognized tenet of the practice. Terisa herself is proof of thatproposition, as the center of her cluster. She, Scott, and Larry haveall been polyamorous since meeting in the Bay Area in the '90s, wherethey were all involved with the same theater community.

Terisa and Scott started dating first. Both were getting out oflong-term monogamous relationships—Terisa had been married for sixyears—and knew they wanted something different. They fell in love, andthough 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 andScott to go with him. "We were like, 'Wow, are we really going to dothis?' " Terisa remembers. "And we sort of just said, 'Well let's jumpin!' "

It wasn't long before they realized there was a thriving community ofSeattleites living the same way. There were local outings, monthly polypotlucks, and a Sea-Poly e-mail list that served to keep everyoneinformed. Larry even found a poly club for Microsoft employees—listedopenly on the company's internal Web site. (Microsoft declined tocomment on the message board, or whether it still exists.) The trio hasbeen together ever since, and they share a lakeside home in Seattle'sMt. Baker neighborhood, where they have a vegetable garden and threedogs. They often go on walks along the lake, hand in hand in hand. "Ithink if we were all given a choice, everyone would choose some form ofopen relationship," Scott explains, sitting in the family's hillsidegazebo overlooking Lake Washington. "And I just like variety," Terisachimes in, laughing. "I get bored!"

The trio have had emotional moments. Scott had a hard time the firsttime he heard Larry called Terisa "sweetie" nine years ago. Larry wasnervous when Terisa began semiseriously dating somebody outside thegroup. There are times when Scott has had to put up with hearing hisgirlfriend have sex with someone else in the home they share. And therehave been moments when each of them have felt neglected in their ownway. But they agreed early on that they weren't going to be sexuallymonogamous, and they are open about their affairs. "So it's not as ifanybody is betraying anybody else's trust," says Larry.

There are, of course, some things that are personal. "Terisa doesn'ttell me a lot of the private stuff between her and Matt, and I respectthat," says Scott. When there are twinges of jealousy, they talk themout—by getting to the root of what's causing the feeling. "It's one ofthose things that sounds really basic, but I think a lot of people inconventional relationships don't take the time to actually tell theirpartner when they're feeling dissatisfied in some way," says Terisa."And sometimes it's as simple as saying, 'Hey, Larry,' or 'Hey, Scott, Ireally want to have dinner alone with you tonight—I'm feelingneglected.' 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'sgoing on in their brains—and between their legs."

Larry and Terisa married last year—with Scott's permission—in partfor tax purposes. Larry owns the house they all live in, and Scott paysrent. Household expenses require a complicated spreadsheet. Terisa,Larry, and Scott all have their own bedrooms, but sleeping arrangementsmust be discussed. Larry snores, so Terisa spends most nights withScott—which means she must be mindful of making up for lost time withLarry. Terisa and Larry only recently began dating Matt and Vera, aftermeeting on Facebook, and now every Friday, the couple bring their sonover to the house and the three of them stay all weekend. Matt willusually 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'sclearly the most delicate part of the equation. Matt and Vera have askedNEWSWEEK not to use their last names—or the name of their child—forfear, even in liberal Seattle, they might draw unwanted attention.Though Terisa doesn't have children—and doesn't want them—she adoresMatt and Vera's son, who calls her Auntie. Recently, the child asked hisfather who he loved more: Mommy or Terisa. "I said, 'Of course I lovemomma more,' because that's the answer he needed to hear," Matt says. Heand Vera say they are honest with him, in an age-appropriate way. "Wedon't do anything any regular parents of a 6-year-old wouldn't do," hesays. For the moment, it seems to be working. The child is happy, andthere are two extra people to help him with his homework, or to pick himup or drop him off at school. They expect the questions to increasewith age, but in the long run, "what's healthy for children isstability," 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 AllenaGabosch, the director of the Seattle-based Center for Sex PositiveCulture. But perhaps the practice is more natural than we think: aresponse to the challenges of monogamous relationships, whoseshortcomings—in a culture where divorce has become a commonplace—areclear. Everyone in a relationship wrestles at some point with an eternalquestion: can one person really satisfy every need? Polyamoriststhink the answer is obvious—and that it's only a matter of time beforethe 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 theyhave any other choices beyond the traditional options society presents,"says Scott. "To look at an option like polyamory and say 'That's notfor me' is fine. To look at it and not realize you can choose it is justsad."