According to the research, open relationships almost always have rules.

That is how it works for Chris and James. Over drinks upstairs at the venerable Twin Peaks Tavern in the Castro neighborhood of San Francisco, they beamed as they recalled the day in June 2008 that they donned black suits and wed at City Hall, stunned by the outpouring of affection from complete strangers. “Even homeless people and bike messengers were congratulating us,” said Chris, 42.

A couple since 2002, they opened their relationship a year ago after concluding that they were not fully meeting each other’s needs. But they have rules: complete disclosure, honesty about all encounters, advance approval of partners, and no sex with strangers  they must both know the other men first. “We check in with each other on this an awful lot,” said James, 37.

That transparency can make relationships stronger, said Joe Quirk, author of the best-selling relationship book “It’s Not You, It’s Biology.”

“The combination of freedom and mutual understanding can foster a unique level of trust,” Mr. Quirk, of Oakland, said.

“The traditional American marriage is in crisis, and we need insight,” he said, citing the fresh perspective gay couples bring to matrimony. “If innovation in marriage is going to occur, it will be spearheaded by homosexual marriages.”

Open relationships are not exclusively a gay domain, of course. Deb and Marius are heterosexual, live in the East Bay and have an open marriage. She belongs to the Church of Jesus Christ of Latter-day Saints and maintained her virginity until her wedding day at 34. But a few years later, when the relationship sputtered, both she and her husband, who does not belong to the church, began liaisons with others.

“Our relationship got better,” she said. “I slept better at night. My blood pressure went down.”

Deb and Marius also have rules, including restrictions on extramarital intercourse. “To us,” Marius said, “cheating would be breaking the agreement we have with each other. We define our relationship, not a religious group.”