Swingers, like polyester leisure suits, seemed to have their heyday in the '70s. But couples who openly swap partners for a night of passionate sex with strangers, are becoming a growing trend in a sort of new sexual relationship revolution -- and those who swing say the rest of us monogamists are missing out.

"Nightline" went inside the top secret world of swingers -- as guests, not participants -- to a highly provocative masquerade party at a hotel in New York City hosted by a group called Behind Closed Doors.

Forget the notion that swingers parties are full of middle-aged folks who are bored stiff by years of marriage. Behind Closed Doors selects its members based on attractiveness and age. The younger the better, and not everyone makes the cut.

The couples said they don't find anything wrong with monogamy, but they were looking for something more exciting and raw.

"Our best sex is with each other," said Sara of Eatontown, N.J., who was at the party with her boyfriend Michael. "We have pretty amazing sex at home when we're alone. When we come here it's a physical attraction, not an emotional attraction."


Michael, a 28-year-old construction worker, and Sara, 24, who works in a doctor's office, have been in a committed relationship for more than a year but they do "full swaps," complete with intercourse, but they refuse to kiss strangers.

"Sex is more of a primal, more of an urge-based," Michael said. "The kissing is more intimate so we like to keep that for us."

National surveys suggest as many as 60 percent of marriages involve cheating. One study conducted by the University of Washington Center for the Study of Health and Risk Behaviors showed that in the last two decades, the number of unfaithful wives under the age of 30 increased by 20 percent and number of unfaithful husbands under 30 increased by 45 percent.

Yet every swinger "Nightline" spoke with said they have a cure for that. They said their relationships are more spicy, more honest and more secure because they swap partners.

"People that are of a certain degree of attractiveness are probably looking to interact and swap partners with other people that are a certain degree of attractiveness so they are a good-looking person," said Nicole Cray, a self-described swing school instructor for Behind Closed Doors. "If you're not a good-looking person, it's probably not the right party for you."

Janel and Stevens of Edgewater, N.J., are both in their 30s and have been together for nearly two years. She manages properties and Stevens is a disc jockey. When they attended Behind Closed Doors' party, they engaged in so-called "soft swapping."

"So we do not engage in penetration," she said. "It's a comfort level. We flirt, we hug, we kiss, but there's nothing much further than that."

Janel said swinging works for her and Stevens because it's something they can do together.

"Think about traditional relationships and how much cheating and lying and deceiving there is how much more disrespectful is it when you do it behind your partner's back," she said. "We're sharing something."

Swingers are part of a change that researchers have noted in younger couples' attitudes toward infidelity in recent years. Younger generations are marrying later, and come to the marriage with habits acquired over years of dating -- among them, sleeping with other partners after the initial attraction wears thin.

Sara and Michael, another couple at the Behind Closed Doors party, said they got into the swinging scene because of a Super Bowl bet they made with each other -- if the Patriots won, Sara earned a threesome with another guy, and if the Giants won, Michael got a threesome with another girl.

"Needless to say the Giants won and I had a week to come up with a girl, found a girl, had an awesome experience and ever since then, every weekend, we've been meeting other couples," Sara said. "We're not the jealous type, but we believe that 'sharing is caring.'"

It usually costs about $200 for a ticket to one of Behind Closed Doors' parties and there's an etiquette involved. Only single women or couples are allowed inside, but no men who are flying solo.

"Women set the rules," said Nicole Cray of Behind Closed Doors. "Men can talk about what they want but women make the decision about what they want to do and don't want to do."

And women aren't just setting the rules, they are driving the explosive growth of the swinger culture on the Internet.

Five years ago, AdultFriendFinder.com, a website that helps swinging couples connect with other couples for a monthly fee, had 5.4 million swinger subscribers worldwide, and now the company said they have more than 10 million. Ryan Hunt, the community manager for the website, said the majority of the traffic to the site is coming from women.

"This is a new generation of swingers," Hunt said. "Usually people get into swinging between 35 and 50+ but we've seen people who are now more comfortable with being online and having kind of a more sex-positive attitude towards things that want to explore swinging at a younger age."

Another website, AshleyMadison.com, lists 14 million people in 22 countries who are looking to have extra-marital hook-ups. Behind Closed Doors also has a website, SchoolofSex.net, that serves as a social network for members to meet each other online.

But there is a reason why the Bible includes a commandment against coveting thy neighbor's wife -- we might be biologically hard-wired to have several sexual partners. Helen Fischer, an anthropologist who has studied sex and adultery for years, said she has lived in 42 countries and there is "not a culture in the world that is not adulterous."

"We are predisposed to eat meat. We are predisposed to fear when an animal is running towards us and we probably are predisposed towards adultery," she said.

Another couple who attended the Behind Closed Doors event was Cassie and Mike from Bristol, R.I. They have been married for four years, both have kids and successful careers -- Cassie runs a hair salon and Mike is a solutions engineer. They look like your next-door neighbors, but they spend the evening scoping out the room for strangers that seem approachable for sex.

"I think the best way to describe [swinging] is 'dating as a couple,'" Mike said. "We find it erotic to go out and meet people that have similar interests and similar sexuality."

"Just makes it hot," Cassie said. "Swinging is not love. I'm in love with my husband and it just enhances out sex collectively afterwards."

"There's a lot of branches on the swinging tree, so you have to figure out which branch do you swing from," she added.