My friend Ann and her girlfriend are having IVF in New York. My friend Hatty is “basting” every month in London with a gay male friend who has offered to help her have the baby she longs for. My mate Shona shacked up with her boyfriend the day she met him, and was pregnant after two months. They all ask: do you think I'm doing the right thing?

What can I say? Except that it's pure luck that I ended up with a nice bloke, two children and a ring on my finger, and I could never judge any of these three for finding their own way to make a family. They are educated, they are solvent, they are mature, they have inner resources that will make them great parents.

So when the BBC recently asked me to make a radio programme about the return of marriage to the centre of political debate, I assumed I'd be taking a pretty liberal line. Experts of all political stripes are agreed that stability is hugely important for children. But stability, I figured, surely came in all shapes and sizes.

The reality nags at me. Analysis of the Millennium Cohort Study, of 18,500 babies born in 2000 and 2001, finds that education, income and age (the higher the better) are important factors in whether two parents will stay together. But the biggest single determinant of stability is whether they are married or not. About half — half! — of cohabitees split up before their child reaches 5. The richest 20 per cent of cohabiting couples do better, but their rate of breakup is no better than that of the poorest 20 per cent of married couples. So while poverty puts a strain on relationships, marriage seems to buffer that strain.

This has made me wonder whether it is a bit of a middle-class luxury to be so reluctant to judge other people's relationships. Is it, in fact, a kind of snobbery in those of us who babble about the liberation of alternative life choices, who know nothing of the ugliness and loneliness of a teenage mother's life on benefits? We like to think of lone mothers as robust martyrs, struggling but winning in quiet, spartan homes. But at the lower end of the scale the reality is often a succession of boyfriends who bring a hugely inflated risk of domestic violence both to the mother and to the child who witnesses it (who is more likely in his turn to become violent).

What we are really doing, when we say that anything goes, is denigrating commitment. And that is a problem. For commitment, experts agree, can make the difference between a happy, well-adjusted child and one for whom life will be much more of a struggle. In fact, lack of parental commitment is a serious barrier to social mobility.

I still don't think it's the ring on the finger that matters as much as the attitudes that seem to go with marriage. Can we bottle these and spread them around? Academics at Denver University have developed a “theory of commitment” that says, essentially, that the best relationships are those in which two people see themselves as “us” more than as “you and me”. They make sacrifices for each other, and give priority to each other's needs. They have found that men who “slide” into relationships, moving in before they get engaged, often remain less committed to the relationship (whether or not they eventually get married) than men who “decide” first that they want to get married and then move in together.

While the women they study tend to see moving in as the point of commitment, many of the men admit they are still hoping to find someone better. When a child comes along, they are more likely to feel trapped than those who can see that child fitting into a lifetime commitment. Maybe this explains why some UK charities now describe children who have never known an adult to put their needs first — a dismal fact that I have found myself coming back to over and over; a selfishness of desperate proportions.

The Denver study is a surprising, modern vindication of an old-fashioned idea. You don't get much support from your peers these days if you ask him not to move in until you're engaged. But human nature does not always move as fast as fashion. The poorer you are, the less you can afford to be prissy. But our reluctance to make a distinction between living together and being committed to each other doesn't do anyone a service.

It's hard to make sense of other people's lives. But our desire to blot out difference makes it even harder. There is less up-to-date research in this area than there should be, because marriage no longer exists as a statistical category. The term “marital status” was abolished in government research in 2003. Everyone is now a “couple parent family” or a “lone parent family”.

Another result of our squeamishness is that the State intervenes mainly to pick up the pieces of family breakdown — crime, drugs, poverty — rather than trying to prevent it in the first place. This leads some people to see these problems as intractable. But that can't be right. Experiments from Bristol to Milwaukee suggest that you can teach people how to live better together, just as you can give antenatal classes about birth.

The paradox is that the more the State tries to intervene, the more it is resented. Normal families don't want to be told what to feed their children. But some people need to be. The Government wants more stability, but it fears stigmatising any group.

But to return to where I began. Shona's boyfriend has left her. He said he “wasn't ready”. She'll be all right. Her own parents — like those of all three of the friends I mentioned — are supportive and are still together. Still living the rather banal suburban existence that we all affect to despise. Stability is so dull. But we need to stop dissing it and take another look at the facts.

— The Blessing of Marriage is broadcast tonight at 8.30pm on Radio 4 and repeated on Sunday at 9.30pm

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