Only You. And You. And You.

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. (Article continued below...)

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."