Threesomes: A User's Guide

To determine if you're ready for a ménage à trois, see if you feel comfortable raising the subject with your partner. You can ask point blank if he's ever thought about it, or you can broach the subject in a roundabout way—through dirty talk during sex, or while watching a conveniently telling movie or TV show (see above).

Regardless, you should then be able to speak frankly about the realities of the situation. Make it clear that you find your partner attractive and that your desire isn’t a result of deficiencies in your relationship. Then be honest with yourself and with him about your capacity to not just endure, but to enjoy sharing each other with someone else.

"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.)