Image caption From top left, clockwise: Sarah, Chris, Charlie and Tom

Imagine one house, with four people, but five couples. How does it work, asks Jo Fidgen.

Charlie is talking excitedly about a first date she went on the night before.

Next to her on the sofa is her husband of six years, Tom. And on the other side of him is Sarah, who's been in a relationship with Tom for the last five years. Sarah's fiance, Chris, is in the kitchen making a cup of tea.

The two women are also in a full-blown relationship, while the two men are just good friends. Together, they make a polyamorous family and share a house in Sheffield.

"We're planning to grow old together," says Charlie.

Polyamory is the practice of having simultaneous intimate relationships with more than one person at a time, with the knowledge and consent of all partners. The term entered the Oxford English Dictionary only in 2006, and such relationships are rare enough that Tom finds himself having to account for his personal situation time and time again.

Find out more Monogamy and the Rules of Love will be broadcast on Monday 19 August at 20:00 BST on BBC Radio 4 Or catch up later on BBC iPlayer

"The number of conversations I've had with peers where I've started to explain it and they've got as far as, 'so, you all cheat on each other' and not been able to get past that. I've said no, everybody's cool with it, everybody knows what's happening, no one's deceiving each other."

If any of the four want to get involved with someone else, they have to run it by the others - all of whom have a veto.

"We can't use a veto for something as silly as, say, personal taste," says Sarah. "If you were dating somebody and I could not understand why you found them attractive, that would not be sufficient reason for me to say, no, you can't see this person."

What is Polyamory? Image copyright Thinkstock The word entered the Oxford English Dictionary in 2006, where it is defined as: "The fact of having simultaneous close emotional relationships with two or more other individuals, viewed as an alternative to monogamy, esp. in regard to matters of sexual fidelity; the custom or practice of engaging in multiple sexual relationships with the knowledge and consent of all partners concerned." Oxford English Dictionary

What counts as infidelity, then?

"Lying," they chorus.

"For example," explains Charlie, "before I went on this first date yesterday, I sat down with each of my three partners and checked with them individually that I was okay to go on this date. Cheating would have been me sneaking off and saying I was meeting Friend X and not say that it was a potential romantic partner."

The rules and boundaries of their relationships are carefully negotiated.

When they had been a couple for just two weeks, Tom suggested to Charlie that they be non-monogamous.

"It was a light bulb moment for me," she says. 'I had been scared of commitment because I had never met anyone I felt I could fall completely and exclusively in love with. The idea of this not being a monogamous relationship allowed me to fall as deeply in love with Tom as I wanted to without fear that I would break his heart by falling in love with somebody else as well."

But how did she feel when, a year into their marriage, Tom fell in love with another woman?

"Well, Sarah's lovely," says Charlie. "I was just so happy that Tom was happy with her."

Sarah's partner, Chris, was less comfortable with the situation at first. They had agreed that they could have other sexual partners, but forming an emotional attachment with someone else was a different matter.

So when Sarah fell for Tom, she agonised over how to tell Chris.

"We sat down and talked about what it meant to be in love with more than one person, and did that mean I loved him less. Well, of course it didn't.

"It's not like there's only so much love I have to give and I have to give all of it to one person. I can love as many people as I can fit in my heart and it turns out that's quite a few."

Previously in the Magazine In interviews, people in open marriages say that although it is not for everyone, it is absolutely possible for adults to be in committed, emotionally satisfying relationships with more than one person at a time. Is it possible to have a happy open marriage?

Chris and Tom bonded over video games and became firm friends. Before long, Chris had fallen in love with Tom's wife, Charlie.

"It had never crossed Chris's mind not to be monogamous - now he says he could never go back," says Sarah.

This quandary over how to manage relationships is something that couples counsellor, Esther Perel, sees people struggling with all the time.

"You can live in a monogamous institution and you can negotiate monotony, or you can live in a non-monogamous choice and negotiate jealousy. Pick your evil.

"If you are opening it up you have to contend with the fact that you're not the only one, and if you are not opening it up then you have to contend with the fact that your partner is the only one."

So how do Charlie, Sarah and Tom handle jealousy?

Not a problem, they insist, and point to a word invented in polyamorous circles to indicate the opposite feeling.

Image copyright Thinkstock Image caption Many people consider monogamy the only option in a relationship

"Compersion," explains Tom, "is the little warm glow that you get when you see somebody you really care about loving somebody else and being loved."

"There's always a small amount of insecurity," reflects Sarah, recalling how she felt when her fiance fell in love with Charlie. "But compare my small amount of discomfort with the huge amount of love that I could see in both of them, and honestly, I'd feel like a really mean person if I said my discomfort was more important than their happiness."

Jealousy has to be handled differently in a polyamorous relationship, adds Charlie.

"In a two-person, monogamous relationship, it's not necessary but it is possible to say, we just need to cut out all of the people who are causing jealousy and then everything will be fine.

"Whereas when you are committed to a multi-partner relationship, you can't just take that shortcut. You have to look at the reasons behind the jealousy."

If an issue does arise, the four may stay up all night talking it over.

The thing about a monogamous relationship, it can give you some sense of certainty and surety, somewhere you can feel safe and at home. Marian O'Connor

"We do so much more talking than sex," laughs Charlie.

But some argue that it is natural for people to bond in pairs.

Our desire for monogamy has deep roots, says Marian O'Connor, a psychosexual therapist at the Tavistock Centre for Couple Relationships in London.

"As children we need someone who loves us best of all in order to thrive. There's normally one main care giver, usually the mother, who will look after the infant.

"The thing about a monogamous relationship, it can give you some sense of certainty and surety, somewhere you can feel safe and at home."

Sarah, Tom and Charlie agree that a safe base is important, but see no reason why only monogamy can provide one.

"I feel safe and secure, with the ability to trust and grow, with Tom, Sarah and Chris," says Charlie. "It is from the base and security of the three of them that I face the world and the challenges the day brings."

"The way I see it, it's only a problem if I feel like one of my partners is spending more time with all their other partners than with me," says Sarah. "It just leads to people feeling hurt."

A shared Google calendar is the answer.

"We mostly use it for keeping track of date nights," says Charlie. "The couple who is on a date gets first pick of what film goes on the TV and it helps keep track of who's in what bedroom."

Sarah chips in. "So, for example, I have a weekly date night with Charlie. It's us snuggling up, us with the TV, us going to bed together and all that kind of business."

I'm preparing for 30 years of being made fun of Charlie

Perel sees polyamory as "the next frontier" - a way of avoiding having to choose between monotony and jealousy.

"We have a generation of people coming up who are saying, we also want stability and committed relationships and safety and security, but we also want individual fulfilment. Let us see if we can negotiate monogamy or non-monogamy in a consensual way that prevents a lot of the destructions and pains of infidelity."

But it's not an easy option.

"We get funny looks in the street," says Sarah.

"And every time you out yourself, you risk losing a friend," adds Charlie. "I'm preparing for 30 years of being made fun of."

Tom is cautiously optimistic that polyamory will become "average and everyday".

"Anyone who is expecting some massive social change overnight is terribly mistaken, but it will happen."

In the meantime, the four of them are planning an unofficial ceremony to mark their commitment to each other.

"Sometimes people just write the relationship off as a lazy way of getting more sex than you normally would. There are easier ways," says Tom wryly.

They all agree managing a multi-partner relationship can be exhausting.

"But we don't have a choice. We're in love with each other," they chime.

Monogamy and the Rules of Love will be broadcast on BBC Radio 4 on Monday 19 August at 20:00 BST , or catch up with iPlayer