"It is important for couples to talk about the fantasy, because each person may have a completely different picture of what the reality would look like," says Dossie Easton, a San Francisco therapist and coauthor of The Ethical Slut. "By talking about it, you'll find out if either of you have specific concerns about the idea." Do the two of you desire a man or a woman? Will there be full-on sex with the other person? Does everyone have full access to everyone else's… areas?

The answer for Janet* and her boyfriend Jack* was an emphatic no. "Neither of us wanted to be with another guy, and the idea of him watching me and another woman was hot—but the idea of me watching him and another woman was repulsive," says Janet, a 25-year-old historian. "So the rules were that he could watch us, direct us, and do anything he wanted to me. He was quite happy with that."

Making rules about what's going to happen may seem like a spontaneity killer—but that's better than turning the experience into a relationship killer. Besides, there'll be space for spontaneity within the boundaries you set with both your partner and the third person, which should also include the measures you'll take to protect everyone from STDs. (For instance, two women means two different condoms.)

"There's no such thing as too much decision making beforehand," says Jen Sincero, author of The Straight Girl’s Guide to Sleeping with Chicks. "You also need to make sure that both partners feel comfortable pulling the plug in the middle if things start feeling weird."

Which is exactly what happened to Jim* and Tracy*. After she agreed to—and enjoyed—a threesome with another woman, she requested they try it with another man. Believing the situation to be a sort of quid pro quo, Jim agreed despite his misgivings.

"I'm not a homophobe and I'd enjoyed seeing my wife with a woman, so I felt an obligation to say yes to a man," admits the 34-year-old lawyer. But before anyone's trousers dropped, he changed his mind. "Thankfully we had a 'safe' word I could say to stop everything," Jim recalls ruefully. "I had really been hating every second of it, and I knew it’d get to the point where our whole marriage could suffer."When Lesbian Obsessions Backfire On Men

"We didn't really 'choose' the three friends we've been with—it was all more a function of circumstance," explains Jaime, a 29-year-old speech therapist who has been with her boyfriend for six years. "It always started after a party, and it always ended with hugs; it was really nice. But my boyfriend and I had discussed the theoretical possibility of a threesome beforehand. And we knew the people well enough to believe they'd be OK with things afterwards, but not so well that a need to sever ties would have been hard."