It is a pity there is not more crossover between newspaper readerships in Britain, because if there were, we might be more honest about the contradictions that currently attend the country's collective relationship with marriage. Yesterday offered a graphic illustration. While The Times, followed by The Independent and others, reported a call by the most senior family law judge for legal protection for cohabiting couples, the popular press exulted in the still unconfirmed news that Kate Moss was to marry. Her nuptials, to her rock star partner, Jamie Hince, it was said, would take place in July. The country, no doubt still recovering from the 29 April wedding extravaganza, will be invited to wallow in another. I'm pretty sure they won't pass up the opportunity.

Compared with almost anywhere in Europe, and certainly compared with the US, Britons have managed to keep in their mind lavish weddings as the romantic start to a marriage for life, while eschewing the institution in record numbers. You now have a situation where £30,000 (rather more than the average annual salary) is not considered an unreasonable outlay – the castle, the dancing, the long-distance honeymoon – and lack of time and money is then cited as justification for not marrying.

Small matter that a civil marriage costs £77 all in, and you can do it at the local register office in your lunch hour; the ceremony, as much as the substance, has come to be the thing. And with no social censure for cohabiting, and producing children (how quaint this now sounds) out of wedlock, a romantic celebration is mostly what is left.

But it is not quite that simple. A residual idea of substance remains. Survey after survey shows that marriage is the ideal for which most people strive, not least that much-vilified group, teenage single mothers. Marriage is also still regarded, just about, as the respectable norm – as a somewhat chastened, but unrepentant, Ed Miliband might testify after the tongue-lashing he received about his cohabiting status from callers to the Jeremy Vine show last month. People may have qualms about marriage themselves, but when it comes to those who represent them, well, they would really rather they conformed.

So if the Government decides, as Iain Duncan Smith and others would like it to, that it should encourage the married state – because it is associated with more stable families, better prospects for children and wider social stability – it still has something to work with. But it had probably better seize the moment, because the proportion of cohabiting couples rises by the year, and the residual sense of marriage as the norm may not last forever. It is to cater for the wreckage of this rising number of not-married partnerships that Sir Nicholas Wall, head of the High Court's Family Division, made his intervention.

Which is where I should lay my cards on the table. As someone happily married for many years, who embarked on the project pretty much as a joint romantic adventure, I thoroughly recommend it. But I am not indifferent to the argument that marriage produces more stable partnerships only because of the sort of people who take their vows.

If there were no such thing as marriage, then maybe these same individuals would make a success of cohabiting, too. For all the starkness of figures showing that more than one in four cohabiting couples break up within five years of the birth of a child, compared with fewer than one in 10 married couples, that may have as much to do with character and circumstances as it does with the institution. So you won't find me urging Ed Miliband to "regularise" his situation and make "an honest woman" of Justine, though it does seem rather an oversight on his part not to have registered himself as the father of his first son.

Marry or not, as you choose. Where the law is concerned, though, I feel that it should be reciprocal. If two people enshrine their relationship in the law, they deserve its protection – and the converse should also be true. If, on principle, for lack of commitment or for financial reasons, two people decide not to register their relationship, or if they just don't get around to it, what duty should the law have to them? None whatsoever, I would say, beyond what one or other might try to reclaim for themselves, at their own expense, through the civil courts.

Sir Nicholas argues that when long-standing cohabiting couples break up, one – usually the woman – loses out. She may be left with children, no claim on the joint home and no guaranteed means beyond her own earnings and state support. To which I would respond, with hardened heart: you should have thought of that. And if one partner then invokes the mythical protection of common-law marriage, a myth which still – apparently – enjoys currency, I would say: more fool you. How much airtime and newsprint has been spent with authoritative figures insisting that "there is no such thing as common-law marriage". Do we need expensive public-service advertising to spell it out?

So yes, there will be hard cases. But hard cases, as we are constantly told, make bad law. And bringing cohabitation and marriage into statutory alignment – or going a long way towards doing that – only risks new unintended consequences. In truth, it could be seen as punitive, rather than protective in intent, discouraging the richer cohabitee from sticking around. And where would it leave civil partnerships, introduced specifically to give same-sex couples the legal status they sought? If anyone understands the difference between a legally recognised relationship and one that is not, it is those who have striven so hard for that right.

Of more benefit than changing the law would be to strengthen the case for marriage – less for moral than for coolly practical reasons. One explanation for why the institution has declined so sharply in Britain is that the incentives – above all economic, but also social – have been deliberately shrunk by successive governments. Indeed, for those receiving benefits, any incentives are mostly the other way. Reverse that, and let's see what happens. Just refrain from making the argument as a sermon; sell it as self-interest instead.

m.dejevsky@independent.co.uk