To most people, the idea of their loved one dating and having passionate sex with other people is repellent. But for two married couples -- with children -- in Somerville, Mass., and thousands more across the country, this is the happy, stable norm.
Sierra Black, 33, is a writer for a popular parenting website. She's been married to Martin, 47, for almost nine years. Martin is a research scientist at a nearby university. (Martin and others in this article will be identified only by first names.)
Though most nights they're home together to put their two young daughters, ages 7 and 4, to bed, on other nights Sierra might be found canoodling with her lover, Aaron, and Martin might go on a sleepover date with his tall, blond girlfriend, whom we'll call J.
"The thought of Martin having sex with J doesn't bother me at all," Sierra told "20/20." "I know when Martin is with J he is happy and loved ... and I want him to be having those experiences, whether he's with me or with someone else. ... Whatever happens after 7:30 p.m., happens after 7:30 p.m."
"I make [dates with J] the least disruptive as possible," said Martin. "Sometimes, we just go out and have dinner ... and other times I have a sleepover. And I let [my kids] know that I'm having one, so if they wake up at night ... [and] need to go to bathroom, they know that I'm not there, but I'll be back in the morning."
You might think Sierra and Martin's daughters think their parents' arrangement is unusual, but when "20/20" anchor Elizabeth Vargas asked their daughter, Rio, if she thought her family was different from other families, she replied, "Not really."
Rio's definition of an open marriage was fairly precise, for a 7-year-old: "Your parent or one of your parents is dating a different person that's not part of your family," she said.
Sierra and Martin are very close friends with another local couple: Molly, 35, and David, 43. They have an open marriage, too, and are parents of a 6-year-old daughter. Those aren't the only things the two couples share.
Martin's girlfriend, J, is also David's girlfriend.
Molly has a boyfriend named Mark. Molly also has a girlfriend: Sierra. Sierra has another girlfriend -- Romy.
"It's a tad kooky, yeah -- it's unusual," said Sierra.
Said Molly, "I get a lot back from this. I have a tremendous amount of love and support in my life, and that is because I have all these strong relationships."
These extracurricular relationships are not fleeting affairs and the couples aren't "swingers." Though Molly and David have been married for 12 years, Molly has been seeing Mark for five years, and David has been seeing J for three. Molly and Sierra have also been intimate for three years. And friendships were often cultivated years before things got intimate.
Honest, continuous communication is key, say many couples with open marriages.
As Martin put it, "There's no cheating."
Sierra added, "We are committed to being an open book with each other, and it's all based on a really high degree of love and trust."
"They have a very specialized ethical code," according to Dr. Elisabeth Sheff, a former sociology professor at Georgia State University who has studied people in open relationships for 15 years. "There's a real ethical basis by which they manage their relationships. In the end they may even be more egalitarian and kinder than those in monogamous relationships...which are often on auto-pilot."
Dr. Sheff says it's estimated that a half-million Americans have open relationships in one form or another. Numbers are hard to come by since many adults in open relationships do not bother getting married legally.