On an unseasonably cool August Sunday morning in Topanga Canyon, just north of Malibu, a family of four arrives at the Inn of the Seventh Ray, an all-cage-free, everything-local restaurant that's typical of the neighborhood. This brunch is a welcome respite from the errands and worries that increasingly fill their days. Jaiya Ma, the center of the clan, is a 34-year-old with dark, wavy hair and caramel skin. Her life is wide open; she falls in love easily, suffers willingly. Next to her is Ian Ferguson, a thin 44-year-old with a shaved head and a goatee, feeding bits of eggs Benedict to their energetic 2-year-old son, Eamon. Ian and Jaiya have been lovers for four years. Sitting across from Jaiya is Jon Hanauer, an extremely fit 48-year-old wearing wire-rimmed glasses, who serves as Eamon's primary caretaker. He and Jaiya have been in a committed relationship for almost a decade.

They all live together just a few minutes up the hill, in an airy modern house with floor-to-ceiling windows and a view of downtown Los Angeles. It's the kind of place where you'd expect a designer to live, and indeed one does live there: Ian owns Dig It Furniture, which has been featured in Elle Decor. The walls are covered with paintings and collages he and Jaiya have made, and toys are scattered everywhere. The only thing out of the ordinary is the sleeping arrangements: Jon and Jaiya have their own rooms on the ground floor, while Ian rotates between his office in the garage, Jaiya's room, and Eamon's room, where, until recently, one of the adults would sleep every night.

Neither Jon nor Ian is legally married to Jaiya. Both are allowed to see other women. But the three of them live a lifestyle that—much of the time—isn't that different from a conventional marriage. They're one of an estimated 500,000 polyamorous families in the United States. Polyamory, which literally means "many loves," usually isn't about having sex with whomever you want, whenever you want, as nonpractitioners often assume. It can also describe relationship configurations like Jon, Jaiya, and Ian's—governed by rules, responsibilities, and expectations—which add up to a kind of de facto polygamy. The more specific term for their arrangement is polyandry, in which multiple men are with the same woman, a far less common arrangement (both today and throughout human history) than polygyny, in which multiple women are with the same man. Jaiya, who founded a successful sex-education company, is typical of the women in polyandrous triads: intelligent, self-possessed, professionally accomplished. The men, on the other hand, have typically suffered a relationship catastrophe that prompted them to seek radical change. Jon could be speaking for any of them when he recalls, "I knew in my heart that I had to find a different way to love."

It's not an easy lifestyle. "Think about all the challenges of any live-in relationship, squared, and you'll see the problem," says Janet Hardy, coauthor of The Ethical Slut (widely considered the bible of polyamory). By chance, the previous night at a bar in Venice, I met Holly (not her real name), a beautiful figure skater turned model Ian dated while Jaiya was pregnant with Eamon. Holly has a new guy, a handsome rocker named Danny. When I reveal this at brunch, Ian looks bummed; he still has feelings for Holly. But Holly's not a member of his and Jaiya and Jon's "species"—she's monogamous—and dating "out of species" is a problem for Jaiya. "It's one of the primary rules of polyamory," Ian explains, because a monogamous woman will almost always force a man to choose between her and his polyamorous life.

For his part, Jon has seen Jaiya get hurt too many times. "She jumps into things and I'm always cleaning up her messes," he says. "She understands that's one of the ways I express my love for her." Jon hasn't taken nearly as many lovers as Ian—all the dating and the play parties . . . that's not his bag. "You've got to manage yourself or you're going to hurt other people," he says.

Jon takes Eamon in his arms and walks to the restaurant gift shop, where they sit in a corner reading children's books. Jaiya and Ian sit in silence as the sun comes out and warms the patio. After we settle the check, they'll all head home. Ian will retreat to his office and work on design proposals, Jaiya will go to her study to work on the book on oral sex she's writing for Random House, and Jon will spend the day playing ball with Eamon.

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Plural love is having a moment right now. That's thanks in no small part to the increasing acceptance of gay marriage: If two men or two women can get married, why can't two men get married to one woman? In Canada, where same-sex marriage has been legal for six years, a case that's expected to go to the Supreme Court could make our neighbor to the north the first Western country that doesn't outlaw polygamy. Here in the U.S., Republican leaders like Mike Huckabee and Michelle Bachmann have made ominous suggestions that legalizing gay marriage will lead to group marriage. And the Mormon Church, not wanting to draw any undue attention to its past embrace of plural marriage, fought Proposition 8 three years ago, perhaps out of fear that practicing polygamists would demand that marriage equality be extended to them as well.

But it's hard to see the harm in egalitarian, secular arrangements like Jon, Jaiya, and Ian's. In 2001, the Law Commission of Canada issued a report questioning the illegality of consensual polygamy, and last year the Canadian Polyamory Advocacy Association asked the courts for a repeal of the ban on polygamy. In the U.S., too, legal scholars have challenged the merits of limiting marriages to two partners. Elizabeth F. Emens of the University of Chicago Law School questions why eliminating the "numerosity requirement" (one man and one woman) is considered undesirable when so many people are practicing non-monogamy, either secretly (through cheating) or serially (divorce, remarriage). And we certainly have polygamy on the collective brain—witness the popularity of TV shows like HBO's Big Love and TLC's Sister Wives.