Watch the CBSN Originals documentary, "Speaking Frankly: Non-monogamy," in the video player above.

More Americans than you might think are openly involved with multiple sexual or romantic partners at the same time. How is this different from cheating? It's all above board.

It's estimated that 4% to 5% of people living in the U.S. are currently participating in what's known as consensual or ethical non-monogamy, a practice in which partners maintain more than one sexual or romantic relationship with each others' knowledge and consent. For comparison, that means non-monogamy is about as prevalent as the number of Americans who identify as LGBTQ, which is estimated to be about 4.5% of the American population.

"What's unusual today is that in open relationships, people are transparent," Helen Fisher, a New York-based biological anthropologist who studies human sexuality, says in the CBSN Originals documentary, "Non-monogamy."

According to a 2016 study in the Journal of Sex & Marital Therapy, about 20% of single U.S. adults reported that they have engaged in consensual non-monogamy at some point in their life. And if you imagine it's only young, liberal, city-dwellers taking part — think again. It's a cross-section of all types of people. That same study found that age, education level, income, religion, region of the country, political affiliation, and race did not impact the likelihood that someone would engage in consensual non-monogamy.

Consensual or ethical non-monogamy is the umbrella term for many different arrangements partners can have besides monogamy. Unlike swinging or casual sex, consensual non-monogamy is typically a long-term lifestyle with committed partners that requires its own set of rules.

Some people may practice polyamory — having multiple steady partners at once. Others may go for open relationships — a committed relationship where sex with other people is allowed. Others might form triads or quads — committed relationships among three or four people.

"I think of it almost like a menu, a relationship menu," said Dr. Elisabeth Sheff, author of "The Polyamorists Next Door." "Serial monogamy is at the top of the menu, and probably the most popular dish that people order. But there's all these other things that people can order now. And they are."

Bridget and Alex, a couple in Brooklyn, New York, have been together for eight years and married for one. They decided to open up their relationship three years ago. (The couple asked that their last names not be used.)

Bridget and Alex, a married but non-monogamous couple in Brooklyn, New York. CBS News

It started, nervously, with one couple they found. The two pairs went out for a "boozy brunch," and Bridget and Alex ended up going home with them. It was the first time they had engaged with anyone sexually outside their relationship. When Bridget and Alex left the apartment and were waiting for the elevator, they high-fived — the "bro-iest thing ever," they laughed at the memory.

"There was always a point where I felt a little constricted, or like I was curious about other people while still knowing I loved somebody else," Bridget said. "So the fact that we were able to do it, and then we felt so good afterwards, was, yeah, it really had felt like I had just gotten a work-around."

Bridget now has a boyfriend outside of their marriage, and Alex has a girlfriend. They all know each other.

"The way maybe our parents were brought up, monogamy felt like it was necessary. It was just the way culture accepted you. It was the way everyone accepted you," Alex said. "Now things are a little different, things are opening up."

But non-monogamy is still shrouded in stigma by much of society. In a 2016 Pew poll of nine countries, the United States had the highest disapproval of adultery, with 84% of Americans disapproving.

These relationships can also be hard on family members, even if they're working for the partners involved.

"For me, my generation, that was called the 'free pass' or cheating," said Lisa, a mother in New York City who learned that her 25-year-old daughter was in an open relationship through her Facebook page.

"I was worried about somebody forgetting to use protection and bringing disease into her life. I also worried that relationships are difficult and giving somebody an out may give them an excuse to have a permanent out and leave her hurt."

Lisa learned on Facebook that her 25-year-old daughter was in an open relationship. CBS News

She draws a line at the idea of a triad raising her future grandchildren one day — something that happens with some polyamorous families.

"There will be no raising a child with a third person unless it's me there as the grandma," she said.

Those who have made non-monogamy work for them say it's widely misunderstood. Contrary to the popular imagination, it doesn't make life one big orgy. Non-monogamous partners say these relationships require lots of planning, honesty, and above all, open communication.

At a tidy suburban house in Kansas City, CJ George, his wife Brandi George, and Brooke Houston have maintained a polyamorous relationship for more than a year.

"It's a triad monogamous relationship," CJ explained. "We do have the three of us, but, yeah, we don't date anybody else. We don't see anybody else. It's not necessarily an open relationship."

(L-R): CJ George, Brooke Houston and Brandi George have a triad monogamous relationship. CBS News

It's a subject of curiosity when people find out. "I've got all kinds of people asking me, 'How do you guys sleep?' and, 'What do you do for dinner?' and all that other stuff," CJ said.

"We eat food," Brooke deadpans. "That's what we eat for dinner."

"We're not here to just wreck the world and burn it down," said Kalyn, a woman in Durham, North Carolina, who said she has one local partner along with partners in other cities who have been in her life for years.

"I had a weird understanding of polyamory, honestly, because you have all these rom-coms and Disney movies and everything that tells you what you are supposed to aim for in life. I thought that's what I also wanted," she said.

Yet her attempts at monogamy brought her nothing but pain — her previous partners could not handle her feelings for other people, and took it as a sign of cheating and deception. She said that in her last monogamous relationship she was "the most depressed and danger to myself that I had been in my whole life."

"So after many attempts after that, we just dissolved that, and I've fully committed to the idea that if you're going to date me, you're going to understand that I'm going to be polyamorous," she said.

There are signs that attitudes toward more open relationships are changing. A 2016 YouGov survey found that only 51% of people under 30 reported their ideal relationship would be completely monogamous.

"None of us are all suited to one thing. So, if monogamy suits you well, great. But find your own way within that," says Mahdy, a Brooklyn man who has been in a triad for more than seven years.

"Create your own rules, your own way of being. You don't have to follow, you know, this religious model or that social model. You can create what works best for you."