But if plural marriage is ever to gain broad acceptance, it won't be because of Mormon fundamentalists. It will be because of people like Ian, Jon, and Jaiya—affluent, educated city dwellers in mutually respectful relationships. And, indeed, some in plural relationships are adopting an activist mind-set. "We're going through right now what homosexuals went through 30 or 40 years ago," says Matt Bullen, a 42-year-old writer and married dad in Seattle who is part of a polyamorist cluster that encompasses five people and two legal marriages. "We need to start putting photos on the desk of ourselves and our partners together. When I'm out in public with my wife and my girlfriend, I need to say, 'These are my partners.'"

Matt's girlfriend, a 43-year-old filmmaker and actress named Terisa Greenan, goes further, expressing the virtues of her lifestyle in stark terms. "Polyamorous people are just smarter," she says. She dates Matt but has lived with Scott Campbell, a 54-year-old classical-music dealer, for 14 years, and Larry Golding, a 54-year-old Microsoft software developer (to whom she's married for insurance and accounting purposes), for 12 years. (Matt's wife, Vee, also dates Larry.) "You've got to have a certain type of brain that's really analytical," Terisa explains. "There are more people, so you have to be able to look at each problem from many more points of view and communicate for that many hours longer."

Whatever you think of Terisa's theory, it's obvious that those in plural relationships are comfortable flouting convention. Many explained to me that humans aren't hardwired to have just one partner. Mary, a 26-year-old Ph.D. candidate in economics at Boston University (who asked that her real name not be used), says she's known since she was 14 that monogamy was anathema to her. "That's when I realized that maybe it didn't make sense for me to suppress these feelings just because of a societal norm," she says. Hardly an insatiable minx, Mary claims she's "not a sexual person at all" and still lives—in a polyandrous triad—with her first boyfriend.

Ian went plural four years ago, after his six-year marriage fell apart, when he adopted a doctrine of "total authenticity. Anybody I was interested in, I told them right off, 'This is what I'm about.' Within four weeks, I was dating four women at the same time." He met Jaiya at an improvisational-dance class near his house, and they immediately fell for each other. Jaiya got pregnant and Ian moved in. "It was the first time I ever truly surrendered to a man," she recalls. It wasn't until after Eamon was born that Ian made the transition from a playing-the-field version of polyamory to something more like common-law polygamy. Ian would like to be dating more, but fatherhood and the demands of being the household's primary breadwinner have shown him that while "love is infinite, time is not."

Six years before she fell for Ian, Jaiya, who was engaged at the time, met Jon at a tantra class in Cincinnati. A couple of months later—two weeks before her wedding day—they reconnected and hit it off. Jaiya canceled the wedding when her fiancé couldn't handle the competition for her attentions, and she's embraced polyamory ever since. She now uses Jon as a sexual guinea pig in her Red Hot Touch instructional video series. "I tell people I'm an international penis model," he says ruefully.

Most who take part in plural relationships claim not to feel sexual jealousy. Dean, a 26-year-old software engineer in Boston, recalls being disturbed at the start of his relationship with Mary and her long-term boyfriend, Max (all three names have been changed), a 28-year-old intellectual-property lawyer, when he overheard them having sex in the next room. In time, he decided this was a selfish reaction. "Just realizing that there are times when she wants to have sex with one of us specifically makes things a lot easier," he explains. "Knowing that it balances out over time makes it easier too."

The trick, Matt Bullen explains, is being able to ask yourself, "'Why am I happy when my partner is satisfied in any other aspect of life, but then suddenly when it comes to sex it's got to be awful feelings of mistrust?' Can you isolate it so much that it becomes like a little trapped rodent in a cage where you can say, 'See, it wasn't that scary at all'?" He claims it doesn't upset him when his wife uses their marital bed to fuck her boyfriend (and Terisa's husband) Larry. "It's just a bed," he says. Some claim to derive pleasure from seeing their lovers getting screwed at play parties.

But however much they thumb their noses at jealousy, polyamorists can still fall prey to it. Scott recalls feeling stung the first time he heard Larry call Terisa "sweetie." Vee Bullen remembers being distraught when she heard Matt call Terisa "darling." The key, according to author Janet Hardy, is not giving in to your partner's jealousy. "You have to hear your partner be unhappy without feeling like it's your job to fix it," she explains. "Otherwise you're robbing your partner of the opportunity of learning how to survive jealousy."