You believe WHAT?

Terisa Greenan and her boyfriend, Matt, are enjoying a rare day of Seattle sun, sharing a beet carpaccio on the patio of a local restaurant. Matt holds Terisa's hand, as his 6-year-old son 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 the process, they rearrange themselves: Matt's hand touches Vera's leg. Terisa gives Larry a kiss. The child, seemingly unconcerned, puts his arms around his mother and digs into his meal.

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 datingScott, 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%