Many people find it hard to stick to the confines of a monogamous relationship. Which is why in America more and more are now choosing to live in 'group marriages'. Helena Echlin reports

Oberon Ravenheart has been married to Morning Glory for more than 30 years. They adore each other. But when Morning Glory brought home her lover, Wolf, Oberon wasn't angry. "I thought he was handsome, dashing, romantic, roguish, and he reminded me of Errol Flynn," he recalls.

That was 10 years ago. Wolf now lives with them in a four-bedroom house in Sonoma County, northern California, along with Oberon's lover, Natalie. Wolf's wife, Wynter (former lover of Morning Glory and, briefly, Oberon) is a frequent visitor.

The Ravenhearts are devotees of "polyamory", the practice of having a romantic relationship with more than one person (coined by Morning Glory, the term has now entered the OED). Disillusioned with monogamy because of widespread cheating and divorce, growing numbers of people in America feel that three is not a crowd - and nor is four or five. The gay and bisexual communities have been practising alternatives to monogamy for years. Now straight people are catching up.

Polyamory is one solution to the monotony of monogamy. But having multiple lovers does not mean they are casual about relationships. It is just that, as Grant, a 32-year-old computer systems manager from Leicester, puts it, "Polys don't believe that commitment has to equal exclusivity." (He and his live-in partner of five years each have two lovers.)

Far from tearing apart relationships, polyamory can actually hold them together. Oberon explains: "The new partner reignites the flames of passion and these can be turned back into the primary relationship." Before Wolf moved in with them, he and Morning Glory used to have phone sex, after which she and Oberon always felt particularly passionate.

Polyamory is often confused with swinging, but while swinging is about recreational sex, polyamory is about relationships. Polys are more likely to be involved in group therapy than group sex, more often found at supper parties than at play parties. They like to get together and talk about issues such as whether or not to have joint bank accounts with their lovers.

Polyamorists favour a wide variety of relationship configurations (unlike Mormon polygamists). Some, like the Ravenhearts, live together in a group marriage (if they have sex with others outside the marriage, they have a "condom commitment"). Others form a "V," in which one person has two lovers but those lovers are not involved with each other. For example, Harry, a 45-year-old technical consultant from Essex, lives with his wife, her other "husband", and their three children. Some coalesce into a triad, in which each one is romantically involved with the other two (these often comprise two bisexual women and a straight man).

Perhaps the most common model is the dyad, or open marriage, in which two people live together but have other long-term lovers outside the home. Naomi is a 46-year-old lesbian who works in IT customer support. She lives in San Francisco with her partner of three years. She also has a 13-year relationship with a bisexual woman who lives with a man, and a two-year relationship with another lesbian woman. Occasionally Naomi and her various lovers go out to dinner or a film together.

Although some 19th-century religious groups practised polyamory, it did not really get going until the publication in 1961 of Robert Heinlein's Stranger in a Strange Land, a sci-fi novel in which sexual possessiveness lies at the root of war and murder. This book inspired a number of ill-fated sexual communes, such as San Francisco's Kerista community, in which every member had sex with each person of the opposite sex on a rotating schedule.

The poly movement has been driven not only by science fiction, but also by feminism. "Increased financial independence means that women can build relationships the way they want to," says Jim Fleckenstein, director of the Institute for 21st-Century Relationships. Polyamory spread slowly in the 80s, perhaps because of Aids, then blossomed with the help of the internet. Today America has more than 100 poly email lists and support groups. Their emblem, which marks the table when they meet in restaurants, is the parrot (because of their nickname Polly). There is a poly magazine, handbooks, endless conferences, retreats and workshops.

By contrast, the UK has only one poly mail list, UK-poly (currently with about 110 subscribers), and one online news group. Two magazines faltered after just one issue each. It is impossible to tell whether Britain has a paucity of polys, or whether they just prefer to keep a low profile. According to Anapol, Britain is "about 15 years behind America in terms of its acceptance". But Grant, who runs the UK-poly list, has a different explanation. He believes the numbers could be much higher, arguing that British people are if anything more tolerant than in America which is perhaps why British polys are less in need of support groups. "We have a tradition of people minding their own business here. People might disapprove, but they won't try to mess up your life. In America, they might call social services," says Grant. In fact, in the US in 1999 one woman had her three-year-old taken away after she revealed on MTV that she was living with her husband and her lover (she later got the girl back).

For most people, the biggest stumbling block to polyamory is jealousy. But polys try to see jealousy less as a green-eyed monster than as an opportunity for character-building. Kathy Labriola, a counsellor in Berkeley, California, who organises poly support groups, advises patients to do a visualisation exercise to help them pinpoint exactly what triggers their jealousy, mentally watching the unfolding of their partner's new romance from first date to having sex.

Usually people find they have only one or two trigger points. "Realising that you are only jealous of a small piece of the overall picture makes it much more manageable," says Labriola. For some people, the trigger is the thought of their partner sharing their bed with a new lover and the solution is to ask them to have sex at the lover's house or in a hotel. For others, such as Grant, the trigger is the thought of being left at home while their partner is out having fun. The answer is to make sure they schedule their lovers on the same nights.

Allowing your lover to sleep with someone else might seem a super-human feat, but most polys say they have overcome or can manage jealousy. In some ways, if you are poly you have less reason to be jealous than a monogamous person, since if your partner takes a lover it need not threaten your relationship.

In fact, poly relationships are more likely to be eroded by minutiae than by melodrama: problems such as scheduling decent time together and deciding who should take out the rubbish. Quoting a favourite poly axiom, Labriola says: "Love is infinite but time and energy are not." After all, many people have scarcely enough time for one relationship these days. For a while, the Ravenhearts solved the time problem by putting a sleeping schedule on the refrigerator, in which each person spent two consecutive nights with each of their lovers and one night alone (even polys need personal time).

The Ravenhearts have worked out many of the problems that plague co-habiting polys, such as how to share money and space. Each person has their own private account, and contributes to a family account for groceries, rent and utility bills. As for their living arrangements, it helps that they have a large house: "Everyone has their own room and an office as well," Oberon explains. "We've found it essential for people to have personal space." Since they often don't bother with clothes, it also helps that they are all comfortable with nudity (including Oberon's 87-year-old mother-in-law, who has her own cottage on the property).

Living together can be tough for anybody, but particularly for polys. "It is hard enough to find one person, let alone two or three, that you are compatible with in terms of managing money, housework and raising children," says Labriola.

When Jonathan Jonas, his wife Deanna and their son moved in with Martha and Don Moore, and their two sons, each family had their own way of doing things and it was difficult to integrate the two. "We tried to fuse two nuclear households and instead we had a meltdown," Martha recalls.

If polyamory is so hard, why bother? For many, it is a hankering for community. Oberon argues that we have become increasingly alienated, partly because of the 20th century's replacement of the extended family with the nuclear family. As a result, many of us are striving "to create complex and deep relationships through extended networks of multiple lovers and extended families". Many polys believe that children also benefit from living in stable, extended poly families (more adults to share the work). Oberon's utopia is attractive, but if just three find it hard to live together, then his vision may be something of a midsummer night's dream.

A more compelling argument for polyamory is human nature. Since so many people are already non-monogamous, why not develop a non-monogamy that is honest, responsible and socially acceptable? Grant says: "It seems weird that having affairs is OK but being upfront about it is rocking the boat." Polys agree that some people are monogamous by nature. But some of us are not, and more and more are refusing to be shoehorned into monogamy. You yourself may already live next door to Mr and Mr and Mrs Jones.