Not that weekly sexual liaisons with people other than your spouse can't cause marital complications and jealousy.
Molly and David had been married a few years before either had any outside lovers.
"Molly first decided she wanted to date someone else," recalled David.
Before David became acclimated to this new reality, he was "horrified by the idea of the two of them going on a date."
Although non-monogamous couples contacted by "20/20" were careful to explain that open marriages shouldn't be confused with promiscuity, researchers have found that people in open marriages simply put a premium on expressing their sexuality. But this sexual lifestyle is not necessarily passed down to their children.
"Some will certainly choose open relationships because it's happening right before them. But for some it's a pain in the ass," Dr. Sheff said. "Because, really, relationships with one person is so much easier."
Molly and David have a daughter. But what kids of open marriages see, Molly said, "is lots of stable relationships with people who chose them consciously and happily."
Clinical psychologist Esther Perel, who wrote about open marriages in her book "Mating in Captivity: Reconciling the Erotic + Domestic," said, "[Parents in open marriages] don't have orgies in front of their children. ... [Y]ou're not giving a message to your children of a romp. You're giving the message to your children that this is an important dimension of life and that you take it seriously and that you negotiate it with care, with responsibility, and with respect."
Indeed, when "20/20" caught up one night with Sierra, Martin, David, Molly, Mark, Aaron, Romy and J, they were having a tame evening, with the three children joyfully playing around.
To those who might criticize such a family life, Molly said, "We all put so much love and effort into this life that we've created for our children, and saying, 'Oh, you're a bad parent' because you've chosen to structure your relationships in such and such a way -- I find that hurtful."
"My kids have every advantage that they would have if Martin and I were monogamously married," said Sierra. "They have a stable, loving home. ... [They have] this wonderful community around them of people who are really just supporting each other."
Could open marriage join premarital sex, interracial marriage, gay rights and easy access to contraception on the list of former taboos now widely accepted in mainstream society?
"I don't think that open marriage will become a dominant model," said Esther Perel. "But it will become one of the many models for relationships. ... [T]here isn't one-size-fits-all."