Modern marriage is, relatively speaking, not fiscally blessed, and an extra few pence in the pound might redress the comparative disadvantage. But the incentive to marry - which is what Dr Habgood announced this week he wants to see - is not the problem. For all the gloomy talk, marriage is not such a minority pursuit. The rate may have fallen away slightly, but numbers of weddings remain close to 300,000 a year. If you count spectators, that makes it, with angling, one of the great unsung British pastimes.
So, unfortunately, is divorce, with the highest rate in Europe, although here also the graph seems to be levelling off - which can probably be attributed to an increase in cohabitation.
At least that's the assumption, in the absence of statistics. Aside from the birth of their children, people living together in stable domesticity barely brush with officialdom except as individuals. Among those who commission social surveys, there has been an apparent reluctance to carry out research on cohabitation, so trends only emerge like photo-fit images. The Office of Population Censuses and Surveys shows a quarter of women aged 16 in 1990 will be unmarried by the time they are 50. That compares with 5 per cent of those who were 16 in 1974. If you discount the idea of late marriage or a national squad of vestal virgins, that suggests a lot of living together - and splitting up.
All this frenetic activity in the market in relationships, married or not, may be fine for the grown-ups. For the by-product - children - it means instability. Ironically, children were once a premium commodity - the very thing that made marriage or a stable relationship attractive. Except for the unusually brave or unusually disadvantaged, marriage was the only framework in which to have children. Gradually, rearing children has slipped down the list of priorities, relegated from first to third place in the Alternative Service Book version of the marriage service, where it now comes after the "companionate" aspects of the relationship.
Economic security was another big sales feature. Hooking someone to provide for you was every sensible girl's aim and if it was someone you could love and respect, so much the better. No sensible girl could now assume that any man would or could provide for her; on current trends, she's more likely to be in part-time work than he in a full-time job. The dynamics of relationships are changing; the increasing emphasis for both sexes on work (or the lack of it) detracts from the value of time invested in the home.
Divorce and separation rates suggest economic stability is not grounds for staying together. Most splits are initiated by women, who are almost inevitably worse off as a result. By contrast, the commodities at a premium in modern marriage - emotional fulfilment and fidelity - have manoeuvred us into a curious paradox.
There's not much left to shock about sexual behaviour. We know the averages - 66 times a year or 70-something in a few frisky areas of the country - and the aberrations. Masters and Johnson's insights have been handed down to a new generation. From tabloid newsprint to cyberspace, there are guides to better loving. With the exception of a few New Age puritans, lack of sexual experience scarcely numbers among the desirable features in a partner.
But once pairs descend into the rabbit burrows of coupledom, all that changes. The partner is charged with being the sole resource for gratification of mind, body and soul. The whole movement towards a more enlightened view of relationships,begun in the 1960s and 1970s, has not lifted the burden from individuals but increased it. We have seen the world; now we expect one person to provide it all.
Dr Habgood's remarks came in response to a survey of 1,000 people, commissioned by BBC Wales, on attitudes to partnership. The results illustrate the paradox. One in eight men and one in 16 women admitted they had been unfaithful to their current partners. Working on the premise that people are as likely to be honest about this as they were about voting Conservative just before the last election, the true figure is surely higher.
In their study of adultery, Janet Reibstein and Martin Richards of the Centre for Family Research at Cambridge University deduced from their research that at least 50 per cent of married people have had affairs. There's no reason to suspect the figure is lower among cohabitees.
And yet the most striking thing about those interviewed was that they continued to believe in monogamy. Sexual fidelity is still a prime commodity, especially among the young, where the shadow of Aids sharpens its appeal. Failure to live up to the ideal is a source of disappointment and disillusion. It may be grounds for rejection, revenge or separation. In the search for the transitory ideal, a small but growing number opt for serial monogamy - and maximum upheaval all round.
Honesty and openness still get top billing as ingredients for a successful relationship, yet the BBC Wales survey indicated two-thirds of those questioned would hide their infidelity from a partner. If the blueprint for modern partnership is intrinsically flawed, it's time to ditch illusions about honesty and fulfilment in favour of what's workable.
In a non-religious society, a blend of self-interest, habit and detachment determines whether couples stay together. It's a volatile mix. As the goals for personal happiness become more ambitious, more relationships are set to end in failure. The focus of commitment is currently towards the adults. Where there are children, the focus of enduring commitment should be to them.
For a lucky few, parenting and personal fulfilment may run happily together, but for many people, they do not. In calling for preferential tax terms to encourage marriage, Dr Habgood is missing the point. By concentrating on the couple relationship, he's running after a fast-disappearing bus. The contract we should be concerned about strengthening is that of parenting, rather than of coupledom. In recognising that, the much-reviled Child Support Agency was founded on the right principles, but with the wrong methods.
Outside this public dimension, everything else is private. If we seek stability in relationships, we could learn a little discretion, a little hypocrisy even, from the Europeans. Perhaps it is time to put away the indulgence of honesty, to turn a blind eye, to accept that love - in the late 20th-century sense - and marriage don't always go together, and to live with it; time to separate family responsibilities from personal rainbow- chasing.
The author is the presenter of the `Money Programme'.Reuse content