I think I’m performing one of my ideological U-turns on the subject of marriage. (If I were a lefty man, I’d score a fat advance and write one of those books offering up the dazzling insight that one becomes more right wing as one gets older.)

The Office for National Statistics (ONS) released figures last week, collated from six years’ worth of government data on family life, which showed: that couples who cohabit are dramatically on the rise; that their children do less well at school, leave education earlier and have a higher risk of developing a serious illness; and that children thrive if their parents are married.

These are facts and despite what one might think about marriage being too old-fashioned an institution for the 21st century, or being a meaningless piece of paper, or representing, as Samuel Johnson said, “the triumph of hope over experience” (or “of imagination over intelligence”, according to Oscar Wilde), the figures speak for themselves.

Married women are healthier than cohabiting ones. Lone parents run a higher risk of developing a long-term illness – although if long-term depression counts as a serious illness, then so I would say are unhappily married women who stay in the union, dying a little every day. Marriage is good for families and cohabitation is not. What are we to make of this piece of David Cameron-pleasing information, given that cohabitation has increased by 65% over the past decade?

Of these cohabitees, childless couples comprise the largest group – there were 1,335,208 couples living together without children, compared with 854,596 10 years before – but there was also a 73% increase in the number of families where the parents are not married, to 909,816 last year. By 2014, according to the ONS study, married couples could account for less than half of British families.

I’ve been married, I’ve been an LAT – “living apart together”, which was very modern of me and pleasingly unclaustrophobic – I’ve cohabited, at length and more than once, and I’m now a single parent again (although a fortunate, fully nannied-up one, which stretches the definition somewhat).

I would say this: all children, regardless of age, race, religion or background, are innately, profoundly, unimaginably conservative and want their parents to be together, both physically and in an official sense. This ambition doesn’t peter out in teenagehood, although it becomes more elastic in what it is able to encompass – my teenage children were constantly badgering me to get married to their sister’s father; I myself took my former stepfather’s surname as a teenager because it made me feel secure.

A great deal is written about how divorce can have a devastating effect on young families, but less is said about the trauma of one’s parents divorcing when one is an adult. Whether you’re young or old, the clearly demarcated world you thought you knew and trusted comes crashing down and it isn’t nice.

This has always been my admittedly slightly defensive, once-bitten objection to marriage: what if it ends? What about the fall-out? Easier to “just” live together and take each day as it comes, without any great public declarations, without inviting an audience to witness your devotion, without asking people to buy you presents to celebrate your glorious and most especial love. All of that is nice – chuck in a big frock and a big party and there’s a part of you that would do it once a month – but it’s wince-makingly embarrassing when, a few years down the line, the sacramental union blessed by the good Lord turns out to have been a bit of a mistake. Oops.

That embarrassment doesn’t exist if you were merely cohabitees and you can spare yourself the stress and expense of divorce lawyers – an especially unpleasant breed – to boot.

Cohabitation is also to do with self-preservation: men want to preserve the impression that they are still thrusting young bachelors, or that thrusting young bachelorhood is only a suitcase away; and women, naturally warier if they are over 30, want to make absolutely sure they’re with the right person, especially if there are children involved (here’s a tip: if neither of you want to get married today, you still won’t want to get married in three years’ time, which means either you’re just not into marriage – improbable, if you’ve never done it before – or you’re with the wrong person and in denial about it because it’s not actively terrible and it’s comfortable. Old slippers are comfortable too; it doesn’t mean you have to pad around in them 24/7).

There is no doubt that cohabiting is an easier option than marriage – and a more modern, which is to say selfish, one in that it keeps its options open. That’s marvellous if you’re 25 and childless, but less impressive when you’re pushing on a bit and have kids. The idea of commitment may be terrifying, but surely there comes a time when you’re old enough to bite the bullet and get hitched. Or, indeed, ditched if that’s the better option. Cohabitation is fundamentally adolescent – it’s not what grown-ups do.

Having said all that, if we are to take it as a given that marriage is the best possible option for families, the “best kind of welfare”, as Cameron (rightly, I think) told the Conservative conference last week, as well as the thing that children are most desirous of, I do think that we need to reinvent it.

Because it may be true that marriage is good for you, but it’s also true that we’re bad at it. The divorce rate may be at its lowest for 22 years (because if people aren’t getting married, they can hardly get divorced), but one in three marriages still hits the ropes. This is, surely and crashingly obviously, to do with the fact that people find the idea of decades-long monogamy fairly suffocating.

Last month Gabriele Pauli, a German politician, called for “marriage expiry dates”.

“The basic approach is wrong . . . many marriages last just because people believe they are safe,” she said. “My suggestion is that marriages expire after seven years.” The contract would be renewable if the marriage was thriving and would simply expire naturally if it were not.

I think this is a brilliant idea. Let’s talk about marriage, by all means, and praise its many and undeniable virtues – but if we want to encourage people to do it, it might be time to lose the rose-tinted specs and have a hard, realistic look at how best to help it to survive.

india.knight@sunday-times.co.uk

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