The Office for National Statistics, in its deadpan way, dropped a bombshell this week with its revelation that the marriage rate is the lowest ever, or at any rate, in the past 144 years. It was one of those times when it dawns on you that we've parted company, in quite fundamental ways, from the habits of the past.

A couple of generations ago, it would simply not have occurred to anyone that marriage could go out of fashion. Marriage was synonymous with being grown-up and heterosexual, and though obviously people engaged in extramarital sex and had extramarital children, this was a matter for censure or compassion or a rebellion against convention. People married and gave in marriage by way of being normal. Back then, even progressives got married after living together, people like John Lennon who fancied themselves as subversive; now even the Queen's granddaughter cohabits and doesn't care who knows it.

Just under 23 men in every thousand and just over 20 women got married last year. And of them about 45 per cent will end up divorcing, if present rates continue. Claire Tyler, the chief executive of Relate, observed: “According to recent official data, two thirds of the population feel there is little difference socially between living together and being married... Those who marry today tend to be older and wealthier. It seems that society is separating along socioeconomic lines and the common experience of marriage no longer exists in the way it used to.”

Ms Tyler was not, I think, trying to frighten the horses but she scared me. Because the truth is that marriage is coming perilously close to being a matter of class, along with church attendance, home cooking and male employment. This was never so before. As Iain Duncan Smith, the former Tory leader, never tires of pointing out, one of the defining characteristics of the underclass is that its members do not marry - that requires a degree of commitment, of emotional and financial stability. Think of the difference between Shannon Matthews' mother with her several children by different fathers, and her grandparents, for whom marriage and jobs were the norm.

It matters. Marriage provides a kind of psychic security for the people in it. Living in a relationship that is, at least in principle, permanent and exclusive means that there's a security as you grow older that the unmarried don't have. And unmarried women being more prone to suicide suggests that you lose that ballast at your peril. There are the obvious, proven, statistical advantages of marriage - you live longer, you're healthier, your children are better educated and happier - but the real benefit runs deeper. It demonstrates we can make binding commitments.

Did I say we've never been here before? The Emperor Augustus took a dim view of the flight from marriage. His solution was a tax on bachelors.

I think he was on to something.

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