ANDREW LEONARD has shared his bathroom, money, holidays and dirty washing with four different women in the past 13 years. He would say he has been in love; he describes the relationships as like little marriages. But he has never promised to sit beside the fire with any of his girlfriends in old age; he never says 'forever'.

Andrew, a 34-year-old quantity surveyor, argues that you only have to look at the statistics to see why. 'So many marriages break up, which seems a very dark option. And by the time you can look back at three people, all of whom you have loved, and can see why things ended, keeping a distance starts to make sense.'

Couples who live together before marriage run a higher risk of marital break up, a recent survey based on the General Household study reveals. There may be no causal connection - divorcees and cohabitees alike tend to be non-churchgoers and to live in cities; but it is their modern morals that are gaining ground: the majority of couples marrying this weekend will have lived together first.

So Andrew may just be more self- aware than the rest of us when he explains that the easy availability of relationships makes commitment to one person seem awesome. 'Intimacy comes so quickly. You meet someone, sleep with them, start living with them: you never dare ask yourself, 'Is this the one?' because circumstances are so tempting. It would be like thinking about killing someone while holding a loaded gun. So there's a negative frame of mind, preventing you from ever getting any further.'

Increasingly, it seems, even people who do say 'forever' may well have several serious, living-together relationships in the course of their lives. Divorces are rocketing (37 per cent of recent marriages are expected to end before death); cohabitations have an even higher breakdown rate. But people go on plunging into new relationships: 26 per cent of those marrying now are doing so for the second or third time, and even more divorcees live with a new partner without marrying. Zelda West- Meads, of Relate, believes 'by the next century, it won't be at all uncommon for people to have two or three marriages in their lifetime.'

We seem to be moving into a period of de facto serial monogamy. It is no longer rare for an individual to spend his or her twenties with a person met at college, the child-rearing years with someone else, and the years after the children have left home (one of the commonest times for divorce, says

Zelda West-Meads) with someone else again.

Some people are now acknowledging this and not even asking that love should last forever. 'Why have one good thing in 10 years, when you could have three?' asks Susie Warner, a journalist, aged 34. 'Most relationships have a natural shelf-life,' says Henry Ellis, a 36-year- old management consultant, who recently left a relationship of 12 years to live with his new girlfriend. 'You reach a point where someone has nothing new to give you; I imagine most people would probably go in for serial monogamy, if it weren't for children and financial constraints.'

Even children aren't necessarily a deterrent. Annie Dobson, 40, recently had a son with her current partner of four years. 'We both have a lifelong commitment to Daniel, and we're committed to remaining friendly towards and thoughtful about each other because of him. But I don't expect our love affair to last for ever. Boredom has always ended my relationships before. There's nothing to equal the thrill of meeting a new person, the erotic joy of discovering how intimate you can become with a stranger. And if I hadn't been prepared to look after Daniel on my own, I wouldn't have had him.'

Serial monogamy is not, of course, an exclusively late-20th-century idea. When death claimed many more women in childbirth, and most people much younger, it was not uncommon for people to have more than one marriage. The difference now is that people are choosing serial monogamy, rather than having it chosen for them.

In 1871, only 17 per cent of divorce petitioners were manual labourers; the balance has now shifted to the point that unskilled workers are the most likely group to divorce. The wider availability of divorce is clearly not the whole story, however. 'I was brought up to marry a professional and live happily ever after in a nice house, but I decided very early on that I wasn't prepared to accept marriage as a career move,' says Annie Dobson. Women can increasingly afford to support themselves outside marriage; in the past, says Duncan Dormor of the research organisation One Plus One, marriage was the only possible route out of the parental home for most women, and there was nowhere else to go after that.

The biggest factor in the rise of serial monogamy has undoubtedly been changing perceptions of relationships. In a 1955 survey on expectations of marriage, couples stressed the efficient fulfilment of the roles of breadwinner and homemaker as the most important thing in marriage; by 1970 the most important thing was for partners to like each other. Relationships moved away from the idea of service to others, and became something we consume; individual growth and development are no longer so easily sacrificed. 'Since all relationships must have their ups and downs, when the downs come they tend to confirm that moving on is the right thing to do,' says Andrew Leonard.

Contrary to the idea that the increasing numbers of switchback, merry-go- round partnerships denote a flippancy about relationships, Malcolm Wicks, director of the independent Family Policy Studies Centre, believes they indicate greater idealism. With the collapse of belief in heaven, people are increasingly anxious to be satisfied on earth. Marriage is now expected to fulfil a much greater range of needs - emotional, social and physical.

Sex has probably got more to do with serial mongamy than most of its adherents like to admit. 'Who was it who said if you put a marble in the sink every time you have sex in the first year and take one out every time subsequently, you will always have marbles in your sink?' asks Andrew Leonard. 'I'm sure when sexual passion starts to drift, it becomes easier to split.' Annie Dobson says she would rather 'meet a new, mysterious man than almost anything, and it's tantalising to think that that's constantly available, even at some later stage.'

If current trends continue, half the female population of Sweden and Denmark, and a third of much of the rest of western Europe, is unlikely ever to marry. Thirty per cent of British births now take place outside wedlock, half of them to cohabiting couples. This raises the intriguing possibility that the relationship between mother and child might become the one indissoluble union; a matrilineal society, in which women and children move on from man to man.

Leslie Kenton, a writer, has had four children by four different men. She remains on determinedly good terms with her ex-lovers; until recently, two of the fathers always spent Christmas with the family. 'Men have always had to understand that my commitment to my children comes first. But my relationships have never been casual, always tremendously passionate and intense, and they have always continued as long as they went on being fruitful - usually between three and five years. As a child I always found it curious that people got married - marriage didn't seem to serve development or freedom. I always wanted to be sure I was in relationships because they were of value, not because they were paying my bills.'

This suggests a certain toughness of spirit; and Dr David Nias, a relationships psychologist at the Royal London Hospital, confirms that the people serial monogamy suits best tend to be outgoing, social and impulsive, the sort who are also high consumers of other experiences - 'work, holidays, pastimes'. Relationships, for them, are not a refuge. Andrew Leonard refuses to think about getting old and unattractive - 'that seems an incredibly negative way to look at things.' And though Leslie Kenton had three children by the time she was 28, and no career training, she never seriously doubted her ability to support herself and her children.

Such people may show the way ahead. They acknowledge what few want to believe, but statistics prove: that relationships don't always last forever. They seem to have constructed a rather civilised way of life: devotion while passion lasts - whether that's for three or ten years; firm friendship afterwards. The only problem, says Andrew Leonard, is that 'rather like serial killers losing all sense of right and wrong, you can lose a sense of what matters most about relationships.'

Yet even as serial monogamy takes hold, the idea of 'forever' still grips most people's imagination with an enormous tenacity. A 1990 Europe-wide study revealed overwhelming rejection (including by 81 per cent of Britons) of the idea that marriage was outdated.

Alison Talbot, a 25-year-old civil servant, is about to marry. 'I don't see marriage as something which may or may not work out, but as a huge, lifetime's responsibility,' she says. Joel Lacey, 27, is marrying next year. 'I've made a decision not to carry on with serial monogamy, which I see as a bit like hiring cars instead of buying one - if you know you're not going to keep it forever, you don't treat it very well. Serial monogamy is often just convenient - sex without complications. But actually committing your life to someone is not convenient at all; it's an acknowledgement that someone else comes first.'

Besides such passionate ideals, serial monogamists can look a bit selfish. The idea of a lifetime's partnership which is satisfying and strong enough to survive whatever gets thrown at it is noble, exciting and worth hanging on to. So perhaps the really incredible thing, given what we expect of relationships, is that 60 per cent of marriages survive: presumably, quite a few of them must be happy.

(Photographs omitted)