This doesn't mean that Australians are falling out of love, or that they've lost the urge to cohabit. Far from it: "serial cohabitation" is a concept you'll need when you begin exploring the new demography. The rising generation's strongest commitment is to the idea of postponed commitment, and they've made quite an art form out of keeping their options open, hanging loose, waiting to see what might happen next.

Although some ABS estimates suggest that, in the future, as many as 40 per cent of us may never marry, that still leaves a majority who probably will. Marriage is still popular: those who take the plunge enjoy it so much they're increasingly inclined to do it more than once. In 1998, one-third of all marriages were second or subsequent marriages for at least one of the parties. ("Repartnering" is another word you'll need.)

As the rate of marriage declines, we are investing correspondingly more money and effort in nuptial celebrations. Full-scale white weddings are back with a vengeance, even if the symbolism has undergone a radical shift since white was first adopted as the bridal colour. Although only half today's weddings are conducted by a minister of religion - down from 90 per cent in the late '60s - being married in a fashionable and/or picturesque church is still a priority for some couples, to the teeth-grinding irritation of the officiating clergy.

Pre-nuptial agreements are no longer regarded as weird. As we adjust to the idea that about one-third of all marriages will end in divorce, couples are learning to temper their romantic dreams with a bit of cold realism: let's decide in advance how we'll deal with a split, if it happens, because, when it happens, we might be a tad less accommodating than we are now.

You don't like the sound of all this? Then play the marriage game by your own rules, by all means, and encourage your children to do likewise. (Good luck.) But don't forget that marriage carried some heavy baggage through the 20th century. Stable marriages - often maintained at great personal cost - were regarded as the cornerstone of a healthy society, and "getting married" was the goal of many people who feared that to remain single might be interpreted as a sign of some personal inadequacy. (The fact that about 25 per cent of us never married was conveniently ignored.)

However you choose to interpret the plummeting marriage and birth rates, and the soaring divorce rate, marriage is clearly undergoing one of its periodical transformations, for at least three reasons.

First, our lives are too long and relationships too complicated for marriage to work in quite the same way it did when life expectancy was lower and gender roles were more clear-cut.

Second, young people have made a logical leap to the conclusion that one sure way of avoiding the trauma of a divorce is to skip marriage entirely.

Third, the idea of marriage-as-institution is being replaced by marriage-as-relationship. (As Groucho Marx put it: "Marriage is a wonderful institution; then again, who wants to live in an institution?") This changes everything. Glitches that might once have been tolerated as part of married life - from a tiff over money to darker periods of mutual loathing - can feel like potentially fatal flaws when "the relationship" is on permanent trial.

In any case, we now marry - or avoid marriage - for many more reasons than we did in the days when marriage was an acceptable passport to sex, or respectability, or security for women.

But some of the old imperatives still apply. The prospect of parenthood propels many couples into marriage (though about 30 per cent of babies are now born to unmarried parents). Some couples believe the commitment to a formal, legal marriage is an important declaration of intent; others cling to the idea that marriage is a religious sacrament. But, for a growing number, all such considerations have lost their meaning.

Like every generation before them, young Australians will make marriage in their own image. It is already clear they will be less responsive than their parents and grandparents were to the idea of gatekeepers who issue permits: they want the gates into and out of marriage - however that state may be defined - to swing more easily, and less noisily, on their hinges.