It's not 'swinging' and it's not about cheating on your partner. But could it spell the end for monogamy? William Leith talks to 'polys' - the couples who have agreed to share their love with significant others

I'm talking to Danella Shea, a polyamorist from Medford, Oregon. Danella is 53, and runs a pre-school centre with her husband, Michael, 45, also a polyamorist. Danella and Michael are, they tell me, happily married, but they also like having sex with other people, and so, with each other's permission, they do. That's polyamory. But when I say 'having sex', I might be missing the point. What Danella and Mike want is more love and intimacy than they can get from a monogamous relationship.

They are not interested in being unfaithful. Neither do they want to be promiscuous. What they like is having a relationship. They like relationships so much, you might say, that they want to have more than just one. 'This is not swinging,' says Danella.

'It's not polygamy,' says Mike. People get confused between polygamy and polyamory. This is not about one male and three females.'

If monogamy is, as the psychotherapist Adam Phillips says, our secular religion, polyamory is the latest heresy. In a way, it is infidelity without betrayal - or, at least, without the most obvious sort of betrayal. But in another way, it's not like being unfaithful at all, because its practitioners, at best, actively want their partners to take other lovers. Sometimes they want to join in, too.

Hold on a minute, though. This is not like the classic male fantasy of the threesome, which comes from a promiscuous mindset. Threesome fantasies are about having more sex. Polyamory, as polyamorists are fond of saying, is about having more love.

When you talk to polyamorists, they sound strangely calm and beatific, like mountaineers or mathematicians sometimes do - people who have grasped at something fiendishly complicated and scary, and rendered it simple and safe. Frankly, they do not always sound believable. But then, I come from planet monogamy. Heresies such as polyamory, quite naturally, make me feel edgy and defensive.

I mean, what would happen if people could have more than one relationship, and nobody really minded? The world as we know it, surely, would collapse. Imagine coming home and finding a note from your wife saying that she was sleeping with one of your mates. It would drive you nuts. But what if you could sleep with your mate's wife at the same time? That would drive you nuts, too. And him, for God's sake. And what about everybody's kids? What about the jealousies and time management?

These are my initial thoughts on polyamory. And, as it turns out, they are more or less everybody else's initial thoughts, too. Michael and Danella ask me to be sure to change names other than theirs, because some of their polyamorous partners are not 'out', and they worry about censure, not to mention issues of child custody. Like many polyamorous men, Michael is 'out'; like many women in the same situation, Danella is not. As always, when it comes to modern sex, men are in a less precarious position; they have less to lose.

Danella, from an Italian family on the east coast, is only partly out of the closet; her family don't know. 'My mother is still alive,' she says, 'and if she found out, she'd probably say, "to each his own". But my brothers would probably hound the hell out of her.' People tend not to understand the concept of loving more than one person. It messes with the whole scheme of things. 'They see sexual intimacy as something that should only take place between single partners,' says Danella.

When it comes to partner-sharing relation ships, evolutionary psychologists, in one respect, agree with Danella's brothers; it's not the way of the world. As David Buss, professor of psychology at the University of Michigan, and a leading thinker on the culture of human mating, points out, 'there are no cultures in which men are not sexually jealous'. Anthropologists, perhaps driven by idealism, have sometimes believed that some societies encouraged partner-sharing; subsequent research has always shown this not to b true. For instance, the Marquesa Islanders were touted as an example of a polyamorous society until it was discovered that everything depended on men giving permission to other men to sleep with their wives; without this permission, the woman was beaten, and sometimes killed.

Similarly, it was once thought that the Inuit shared their wives liberally; it turned out that this happened only in rare situations; Inuit men it seemed, were as jealous as men everywhere and frequently killed their wives in jealous rages And if you think about it, this makes evolutionary sense; until the age of DNA testing, men could never be absolutely sure that the children their female partners bore were their own; the evolution of male jealousy is a brute fact. Polyamory then, is not a natural state. As David Buss put it: 'there are no paradises populated with sexually liberated people.'

But if partner-sharing is not a natural state, nor is monogamy. This is where evolutionary psychologists part company with Danella's brothers. Faithful pair-bonding for life between two people was not the norm in the Stone Age, and, arguably, it's not the norm now, in an era when half of marriages end in divorce. A brief look at Darwin tells us why.

In order to replicate his genes, the best thing a Stone Age man could do was to impregnate a woman, and provide resources to protect her and her offspring, while trying to impregnate as many other women along the way as possible. And how much have we evolved since the Stone Age? Possibly not at all, genetically speaking. As David Buss puts it, 'The picture is not a very pretty one, but humans were not designed by natural selection to coexist in matrimonial bliss. They were designed for individual survival and genetic reproduction.'

Seen this way, monogamy is a form of civilisation - an attempt to control the rampant promiscuity and gender warfare that is man's natural state. And it has its advantages - it creates family structure, naming systems, and accountability. It's a response to the invention of property and money, too. Monogamy is an enabling tool for the inheritance of goods. So maybe it's not natural, but simply useful as a way of bringing order to a chaotic world.

And that's what's interesting about polyamorists. Talk to them, and the thing they stress, above all, is the importance of order, of 'boundaries'. As Michael tells me, polyamorists often rank their lovers according to a hierarchy. 'People talk about having primary, secondary, and tertiary lovers,' he says. 'Primary often includes sharing finances. Secondary might mean living together. Tertiary might be seeing someone once a month.'

'These are things that get discussed,' says Danella. 'You have to set up boundaries. For instance, safer sex.' Some polyamorists create 'relationship documents', detailing who they've been with, when, and in what circumstances. Michael and Danella tell me that, since they moved to Medford from San Diego in the last few months, they've been 'dating' other couples in the area, but nothing has happened so far. One couple made it clear that they were only interested in a 'quad' relationship - when both couples find their counterparts attractive. But there was 'not enough compatibility.' So, at the moment, as a couple, they're single. Nan and John, both 48, a middle-class couple from New Jersey, are married. Julio, 36, and Amy, 45, are both unmarried and each lives nearby.

Nan and John live with their two children - Adam, 20, and Julia, 17. Together, Nan and John and Julio and Amy form two new couples. At the weekends, John sleeps with Amy, while Nan sleeps with Julio. Amy and Julio are not sexually involved. 'I'm in love with two men,' says Nan, a psychotherapist, 'and I sleep with them both - separately. For me, finding love with someone new doesn't mean finishing my existing relationship with a man I've loved for years, and still do.'

One of the good things is that Nan likes Amy, who works as a pet-sitter. 'She's Jewish, like me - smart, grounded, and sensible. I really warmed to her when I discovered she saves coupons and manages her money like I do. Amy is a far better listener than I am and gives John the attention he deserves. We're close friends and I love comparing notes with her about John. We laugh about the things he does that drive us both crazy.'

John, a lawyer, met Amy at a relationship workshop. 'The attraction was instant and obvious to everyone there, including Nan,' he says. I immediately wanted to be alone with her - there's an incredible sexual spark between us. But we "polys" are just like anyone else - civilised, polite, and appropriate.' John ponders for a second and says, 'People always ask me to compare Nan and Amy, but I refuse. I love them both for being the unique, amazing women they are'.

The biggest misconception about polyamory is that it equals promiscuity. People assume that we have no boundaries. We do - they're just the ones that we've put in place. For me, the golden rule is that we all look after each other. A set-up like ours is a delicate thing, so we need to be gentle with it.'

'There are times when I find it challenging,' says Nan. John, her husband, had been having a relationship with Amy, for eighteen months before Nan met Julio. 'Amy would come to our house and I'd sleep alone in our room while Amy and John took the guest room. One night I went to the bathroom and caught a glimpse of them making love, which made me feel sad and jealous.'

Things changed when Julio came on the scene. 'Sex with John,' says Nan, 'is consistent and sweet; he's an amazing lover. Julio is very different. Our relationship is newer, so it feels more exciting and less certain. He's charming, charismatic and full of energy. We often have sex a few times a day, experimenting with different positions. If I really like one, I share it with John the next time we're alone together.' So does John get jealous too? 'Seeing Nan with her first polyamorous boyfriend was hard at first: I was intensely jealous,' he says. 'But that faded. Not that I don't still have moments of jealousy - it never completely goes away, but it does subside.

'For me, the hardest part of our arrangement is when I feel I'm letting either Nan or Amy down. However organised I try to be, I find life is a juggling act - with work, the children, Nan and Amy - and there are moments when something has to give. It was also very difficult when our children were younger and some of their friends found out about our lifestyle. The idea that our choices were in any way a source of pain to them really stung.'

In many ways, then, polyamory has a lot in common with monogamy - it's a way of imposing some order on the untidiness of human emotions that sometimes works, and sometimes doesn't. But in a polyamorous world, the stakes are higher - there is more scope for things to go wrong.

Like Mike Shea, Julio has always seen himself as a 'poly'. 'To anyone considering becoming polyamorous: think about it very carefully,' he says. People imagine it will be the perfect solution - that they'll get one thing they need from one person, and something else from another. To a certain extent, that's true. You do get twice as much good stuff, but you also get twice as many problems. A situation like ours is fraught with ambiguity and misunderstanding, which can make for a rocky road.'

But this is a rocky road which more and more people are taking. It's also a lifestyle that might have been made with the internet in mind; there are lots of sites in which people chat about their polyamorous activities, tout for new partners, and discuss the welter of new problems polyamory brings. Who, for instance, do you spend Christmas with? What if two partners clash over decor? One woman complains that, since her polyamorous group bought a house together, the two men in the group have become territorial.

There are some problems you can see straight away. Polyamory can be led by the male desire to have sex with more people, but men are often hurt more than they thought they would be when their wife or girlfriend follows suit. You could see that coming, couldn't you? After all, according to David Buss, a man who wants to stray need not be unhappy with his relationship; a straying woman, however, is almost always dissatisfied.

And there are some things about polyamory you might never have thought of. Often, a 'quad' relationship starts well - each man loving each woman, and vice versa. But the women in the group are far more likely, when a bond has been formed, to experiment with each other than the men are to do likewise. 'The men want brotherhood,' Danella tells me, 'but not a sexual relationship. But the women connect more sexually.'

So what accounts for polyamory? You just have to look at the websites: it's because monogamy isn't working like it used to. Peter, a 38-year-old mortgage adviser from Kent, says: 'I have a wife, a girlfriend, whom I love as much as my wife, and a lover. I consider all of them to be a part of my life. If I didn't have this sort of set-up, I would be going around being unfaithful to my wife, and I would have to lie to her all the time. '

For Danella, 'I was married and divorced twice, then lived with someone for seven years. But all of these relationships weren't truly monogamous. The motto of monogamy today is different. I asked one of my partners to consider an open marriage, but he said "no way". But cheating was all right! So, for me, this is about honesty.'

In the end, this new wrinkle in the sexual revolution seems to have been inevitable. We live in a world of affluence; we are always being encouraged to want more. Advertising and body-fascism and celebrity culture make people feel anxious and needy; the constant pressure on our sense of identity makes us want to reinvent ourselves all the time. There's a lot of pressure; no wonder monogamy is cracking.

Danella says, 'have you heard of the Cinderella Complex? How the woman has been programmed to believe that if she only finds the right man, she'll live happily ever after?' And then she says, 'There is no happy ever after.'