A minority group for those who find strength in numbers. A community for people who like their intimate lives communal. Polyamory – the practice of openly engaging in multiple loving relationships – acquired its name in the 1990s, and has been gaining ground as a recognised lifestyle and movement ever since. With the UK's first "poly" website just launched, and Polyday taking place in London later this month, it seems there's never been a better time for "ethical non-monogamists" to stand up and be counted.

Let's clear up a few things first. This isn't polygamy, where one person (usually male) has numerous spouses who cannot pursue other liaisons themselves. Nor is it swinging or adultery; polyamorists aren't into cheating or one-night wife-swaps. It also differs from the hippie notion of "free love", which was primarily about enjoying sex. And it's a bit more specific than an "open relationship". What polys want – not so different from the rest of us – are ongoing, honest, committed relationships. But with several people at once.

The internet has helped them hugely on this quest, allowing individuals who might otherwise never meet to form vast, wide-ranging networks and communities. Through forums, chatrooms and niche dating sites, they can share advice, discuss sexual politics and even find new partners.

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In the UK alone, there are thought to be several thousand enthusiasts. In July, Britain got its first dedicated poly website when Graham Nicholls founded www.polyamory.org.uk. "The whole community was calling out for a main information site," says the 34-year-old artist from west London, currently in a "triad" with two female partners. "It uses podcasts, video interviews and photos, and will hopefully raise consciousness about polyamory in a positive way."

On the internet was where Clair Lewis, 36, from Manchester, discovered the "p" word. "I've been polyamorous since the age of 19, but for the first few years I didn't know there were others like me, or that there was a language for it. There are still no role models; you never see polys in soaps."

Lewis is now national convenor for CAAN – the Consenting Adult Action Network – which campaigns for "the rights of consenting adults to make their own sexual choices, without interference from government", and she regularly uses the internet to make contact with other poly protesters.

Of course, polys don't interact solely in cyberspace. As their lifestyle and beliefs suggest, they want to meet as many folk as possible in the flesh. New York's Poly Pride Weekend offers a picnic, rally and "cuddle party" every October. Britain doesn't yet host such a large or high-profile event, but around 300 attendees are expected at Polyday, a day of talks, workshops, socialising and "sex-positive cabaret" in central London on 26 September.

Organising the day this year is south London-based illustrator Maxine Green, 27. So what can those who attend expect? "The activities include a cartooning workshop and a spirituality discussion, and are divided into three threads: beginners, advanced and open to all," she explains. "We also hope to have an 'agony aunt' session." Cartooning? Yes, enthusiasts will interpret polyamory through drawing.

Not surprisingly, jealousy and time management are perennial Polyday subjects, but another hot topic is politics. "British polys are often into alternative lifestyles and politics, and tend to be more radical and progressive than American polys," says Nicholls. "Some even identify themselves as 'relationship anarchists'."

One such politically-driven poly is Owen Briggs, a 33- year-old gardener from Nottingham. "I believe in trying ' to break down power hierarchies in society, and that means breaking them down in my personal life as well," he says. "If I wish to try to allow others to be free, why would I want to control the people I love and care most about?"

Anarchic approaches to relationships also abound on the "queer" poly scene, which, as Johanna Samuelson and her primary partner Jonathan David explain, is a little different from the standard gay scene. "It's an inclusive, activist community which sees beyond the divide between male and female, hetero and homo," says Samuelson, a 27-year-old postgraduate student from Brighton. "When you start exploring your gender, you may also start exploring your sexuality," adds David, a 26-year-old musician who identifies himself as transgender, and feels that there is less acceptance of polyamory in more conventional gay circles.

Polys often credit the gay-rights movement with paving the way for them in terms of gaining respect and recognition. "It has brought alternative relationships into the public eye," believes Lewis. But many individuals still choose to keep their uncustomary set-ups private. Stuart, 44, Kaye, 43, and Ben, 25 (not their real names), are "out" to close friends and some family members, yet, despite their idyllic and highly domesticated existence – they all live together in west London with Kaye and Stuart's four children, seven chickens and two dogs, sharing bank accounts, the car and a bed – they fear that going public about their three-way relationship could jeopardise their jobs.

They don't, however, hide away. "The kids all find Ben's presence quite natural," says Kaye, "and we go out for dinner and on holiday together. My eldest daughter, who's 19, has coped really well, considering she's a teenager." What about school-gate gossip? "Things have really moved on," she explains. "My children's school no longer makes presumptions about numbers (or genders) of parents. The forms they bring home simply ask, 'Who's in your family?'"

Nevertheless, non-monogamy remains very much a taboo in Western culture, where for hundreds of years our core values have revolved around exclusive pairings and the traditional nuclear family. Polys who are open about their lifestyles inevitably face prejudices. "At times I've felt really isolated and lonely," says Luisa Miller, a 26-year-old event organiser from north London. "People can assume it's just about sex, and having 'fuck buddies'. Despite what you'd think, it's often harder to find relationships, because there aren't a lot of people who are OK with polyamory." David agrees that the poly ethos is too frequently misunderstood: "It gets portrayed as greedy, selfish and over-sexualised."

Males tend to encounter the most suspicion. "There's this perception that it's just a way for men to get their end away," says Nicholls. "In actual fact, the movement has risen out of third-wave feminism, and the first five significant books on the subject have all been written by women."

Maxine Green, who enjoys simultaneous affairs of the heart with three men and one woman, endorses this argument: "My experience of the poly world is that there's much more emphasis on equality than in the average monogamous relationship. Women are just as able to make new connections as men, and are at least as often at the centre of a group."

But not everyone has had such a positive experience. Rosie (not her real name), 32, from Bristol, spent two years in a polyamorous relationship. "Soon after my boyfriend and I got together, we decided to try polyamory, as we often fancied other people and didn't want to limit each other's freedom. It worked well for a while," she remembers. "But I did sometimes have insecure moments when he was off with another lover. The trouble really started when one of my other relationships got more serious, and he became distant and quiet. I was always completely open with him, and constantly emphasised that I was still in love with him, but he couldn't handle it, and in the end we split up."

Rosie sounds a word of warning: "I wouldn't judge anyone for trying it, but I do worry slightly that some people – especially young people – might do it because it's trendy, or because their partner wants them to."

But polyamory is not solely the preserve of those frisky, idealistic youngsters. Pete Benson, 69, has rejoiced in "emotional connectedness with more than one person" for half a century, and last year published his "user's guide" to the practice, The Polyamory Handbook. The American author raised his two children while living in a "quad" with his first wife and another couple in Eugene, Oregon, during the early 1970s. "All five children in the quad family really loved having four parents to love them, pay attention to them, help them, and just do things with them. We adults, too, had more free time by sharing the parenting activities."

Benson and his current wife of six years, Deborah, 56, now share a "secondary partner", Misty, 50. More recently he has started seeing another secondary, Elan. "There is definitely more acceptance now than 40 years ago, when I was in my twenties. Then it was still a socially new thing for unmarried people to admit openly that they shared sex, and it was virtually unheard of for them to live together. Those who openly did otherwise caused eyebrows to rise." And it was called, in what came to be a 1970s cliché, "free love".

Benson welcomes polyamory's tentative moves towards the mainstream. "Nowadays, my wife and I routinely mention being polyamorous when relevant in conversation, as normally as mentioning, say, that we enjoy bicycling." After more than 50 years as a polyamorist, he's a "veteran" in the field. His advice is simple: "It takes thorough communication, flexibility and constant cultivation of one's primary relationship (if you're in such a relationship) to maintain trust. A sense of humour also helps."

Like Benson, Chicago-born medical translator Juliette Siegfried, who lives in Sitges, Catalonia, believes that having more than two parents in a family means "more love, support and financial resources for the child". Siegfried lives with her husband of 11 years, Roland Combes, his girlfriend of two years, homeopath Laurel Avery, and Combes' and Avery's eight-month-old daughter, Maya. Well aware of the discrimination faced by polys, not to mention poly parents, Siegfried, 42, has become something of a spokesperson for the cause, and runs discussion groups in Barcelona, as well as a Yahoo! group, Poliamor, online. "How else will we get past the prejudices?" she reasons.

Combes, a 42-year-old British web programmer, goes one step further: "While I don't agree with dictating to people how they should live their lives, I feel that if governments promoted and encouraged these types of larger families, all sharing resources, it would benefit society as a whole by putting less pressure on the planet."

Though it's unlikely that state-funded leaflets extolling the virtues of non-monogamy are going to hit our doormats any time soon, polyamory's increasing visibility and popularity suggest that in the not-too-distant-future there'll be a lot more of it about. Whether, as Benson puts it, "poly-style open relationships and multi-adult households might one day be accepted by society as a perfectly normal option for living and loving" remains to be seen, but movers and shakers in the poly world are already doing their damnedest to put this unconventional approach to romance on the map.

Polyday is at Dragon Hall, 17 Stukeley Street, London WC2, on 26 September. For details, visit www.polyday.org.uk

The home-makers: Laurel, Roland and Juliette

Husband and wife Roland Combes and Juliette Siegfried, both 42, have always conducted their partnership "on somewhat open grounds", and last year bought a flat in Sitges, Spain, with Roland's girlfriend, Laurel Avery, 43. In January of this year, she gave birth to baby Maya (pictured with the trio above).

"At first I worried about Laurel's 'exciting newness'," concedes Siegfried, who until recently also had a boyfriend, "but things evolved naturally, and now I feel so delighted with the situation. Just after Maya was born everyone was exhausted and communication went out of the window, so now we have a schedule to balance our caring and working, as we all work from home."

"Love isn't limited, but time is," agrees Avery, who adds that, "Maya will benefit from having three loving parents." She also stresses that, "Communication in a polyamorous household is key – just like in a monogamous relationship."

Combes, originally from Chelmsford, England, comments that, "Despite the Catholic culture, people in Spain are more 'live and let live' about our lifestyle than in Britain, where attitudes are becoming increasingly moralistic." But he believes that acceptance in general will rise, "in the same way that it has for interracial marriage and gay partnerships".

The group are even considering, in time, adding new members to their home. "We'd like to grow the family even more," says Siegfried. "We're good at relationships, and you want to do more of what you're good at."

The activist: Clair (in wheelchair) with (left) Phoebe and Lucy

Clair Lewis, 36, from Manchester, is devoted to "parenting, partnering and protesting". She has three children, and two partners, Lucy McAlister, 31, and Phoebe Tunstall, 25.

"We spend a lot of time all together, though Phoebe and Lucy are not in a relationship with each other. The kids love having a big family, with more than one adult to go to if they have a problem. The only thing I sometimes miss is time for friends.

"Recently I got engaged to both Phoebe and Lucy, which is really exciting. It's not ideal that you can only wed one person legally, though; that's something I'd like to see changing."

A genetic illness means that Clair frequently has to use a wheelchair. "My political activism has come out through being disabled. Lots of people think having a disability means you're asexual and shouldn't reproduce; it makes me angry, but also determined to fight discrimination in a whole range of areas, including poly relationships.

"It is a common misconception that it's only physically and emotionally possible to love one person at a time. Nobody contests that you can have three children and love them all as much, so why not more than one partner?"

The experimenters: Johanna and Jonathan

Johanna Samuelson, 27, and Jonathan David, 26, have been living polyamorously in Brighton for 18 months. Each considers the other to be their "primary partner", but Johanna has been seeing another lover for five months, while Jonathan has had a number of short-term affairs. They've also had a three-way relationship with an "intimate friend".

"I was quite shocked when Jonathan first fell in love with someone else," admits Samuelson. "We've had to work out new boundaries." David, too, felt "insecure and jealous to start with", but now enjoys "being able to go out and play with others while still having a long-term, domestic partner."

Sexually, Samuelson relishes the opportunity "to have beautiful moments with different people", and David "to learn some new tricks to teach Johanna". Transgender David also believes that the additional intimacies "have made me feel more comfortable with my body".

"As long as you do things with responsibility and respect, and communicate well beforehand, the positive energy you get with a new person can be really good for your main relationship," says Samuelson. "People see it as having the best of both worlds. But this hides all the hard work you have to do."