Though it's often been the subject of debate, most women would agree that one of the immutable rules of Being a Girl is Don't Hook Up With Another Girl's Boyfriend. But what if you're also hooking up with the girl?

We're referring, of course, to the growing trend of single gals dating committed, heterosexual couples. As in, hanging out, making out, and engaging in typical friends-with-benefits behavior with not one but two people -- who are usually already in an exclusive relationship.

Not even HBO has tackled this one.

We spoke to a few women who are currently in FWB relationships with couples, and we were surprised by what we learned. While the rules are different for every three-person coupling, the arrangement seems to be getting more conventional than ever. Aside from the fact that the hook-up buddies are three people instead of two, the way they function on a day-to-day basis is not much different from the established two-man model.

One of the first logical questions on the very long list is, how does a girl find herself in one of these wacky setups?

Like a Book Club, but With More Nudity
For Amanda, a Brooklyn resident in her early 20s, it was a case of actively seeking out a couple. "I became involved with a wine meet-up group for women [interested in] 'the lifestyle.' I hit it off with a woman at those meetings on a flirtatious, friendship level. We began chatting online, and then the wine meet-up group had a 'bring the husband' party. I ended up chatting with the two of them the entire night about everything from film to sex to politics. Towards the end of the party, we started getting closer and moved to a room that was filled with beds."

She continued to see the married couple just like she might a single guy -- going out for drinks and dinners and hooking up. When asked how they individually made her feel and whether she developed an emotional connection to one or both, Amanda explains that the two couldn't be separated. "I didn't have an emotional attachment to either of them, but they both made me feel comfortable. I felt in control of the situation. I felt a little bit spoiled by how much attention they gave me ... They were so open and honest with each other. So in love. Because of this, I can't really separate how each of them made me feel."

Amanda noted that she was the only other person who had slept with her husband. So was the other woman jealous? Nope! The relationship worked, she explained, because "it wasn't entirely sexual. Three people found a connection through words, and sexuality. Not as difficult as it seems to outsiders."

Menage College
Jasmine, a mid-20s Manhattanite who runs and women-and-couples events, broke it down for us. For her, sleeping with an established couple seemed like an opportunity to explore and experiment with people who were like-minded but emotionally mature. However, after attending "nearly every swingers club, erotic event and workshop in Manhattan," she was disappointed in the sleazy environments, often run and controlled by men. Drugs and prostitution were common.

Thus, Swing School was born. Based on her dissatisfaction with the current options available for women and couples, Jasmine started an orientation / screening process for the "lifestyle" that became part of the curriculum at the "School of Sex" (known for their "Behind Closed Doors" parties, hailed as "best swinger club in NYC" and "top party to hook up" by Time Out New York). Through "Swing School," Jasmine has guided countless single women and couples through the sometimes complicated dynamic involved in bringing another sex partner into their relationship.

It's difficult, she admits, because a lot of times it's about fantasy fulfillment for the guy and not for the women. It's the whole "I'm doing this as a birthday present" cliché. It's important to vet entrants to "the lifestyle," Jasmine says, because "many women in couples are not bisexual -- they try to convince themselves that they might be bisexual in order to 1) please their male partner, or 2) 'fit in' in the swing scene, where female bisexuality has almost become a requirement due to the male expectation."

For these reasons she believes it's very difficult for a woman to find a "quality couple" for play and why these relationships rarely last beyond a few encounters. Thankfully, that's why there's a class.

There's No "I" in Threesome
Dixie, another New York 20-something, didn't experience any of that in her first experience with a couple. She met a married couple through a friend who was into the scene. She sought out the arrangement, she said, simply to expand her sexual experience and horizons.

"My first experience with them was incredibly intoxicating," she says. "It was my first time with a woman in any remotely serious sexual way. They both made me feel wonderful individually, and being with them together was mind-blowing. We weren't even having sex -- just kissing and touching -- but I was sandwiched in between two people who I could tell really loved each other, and that was really magical."

Dixie says that, as she'd hoped, the experience made her feel more comfortable with herself and better in touch with her sexual desires. She discovered that she could be sexual with either a man or woman but was left confident that her preference is for men. And, like Amanda, she didn't speak about an overwhelming amount of ground rules or awkward conversations her relationship with the couple. "[The husband] told me that his relationship with his wife was the most important thing to him, and that he would never do anything to compromise it. Beyond that, he said, he and his wife would have discussions about specific activities that might make them uncomfortable or any specific boundaries they wanted to set -- but nothing general."

Boundaries, Boundaries, Boundaries

It's one thing when the relationships are kept strictly sexual, but it's another thing entirely when everybody hangs out like you normally might with a hook-up buddy. Jasmine advises that a girl considering dating a couple set boundaries and have a strong sense of exactly what she wants out of the relationship: "I tell the women to think of approaching things the opposite of the way traditional relationships are typically formed."

Like Amanda, Dixie saw the couple outside of the party, but things didn't go as smoothly. After they'd seen each other several times, they invited her over one evening to join them for dinner at their apartment, and then drinks at a bar. "We went out to a burlesque show," she said, "and the wife leaned over to me and said, 'We're going to leave at the next intermission. We would love for you to join us.' For some reason, the 'us' rubbed me the wrong way -- it sort of made me hyper-aware that I was the third wheel. Even though it was a perfectly normal thing to say in the situation, I decided I didn't want to go home with them that night." She'd been essentially dating the couple, and suddenly felt that she was more emotionally invested than they were.

It doesn't sound wholly unlike the kind of realization a lot of women have in hook-up relationships with men. But if anything, the "community" aspect of these less traditional arrangements can help foster more openness and honesty than you might have in your two-person casual relationship -- albeit with just as much risk involved for jealousy and emotional disconnect.

Which brought us to the question -- would these single women who date couples invite a single woman in if they found themselves in a committed relationship?

"I think it could go either way for me," Dixie said. "As long as I find someone who I love and who loves me wholly, it doesn't matter."

"Ask me when I'm in a committed relationship," said Amanda.

Jessie Rosen writes the blog 20-Nothings -- an account of getting by from 2-0 to 3-0. She reported on -- but did not partake in -- the activities above. She has, however, been sold in a dating auction. Guess how much she sold for.