In the digital era, those looking for soulmates can be brutally clear about who need not apply. There are websites like Blues Match, for alumni of Oxbridge and Ivy League universities only. Then come the smartphone apps: Tinder, for straightforward dating, and ‘BeautifulPeople’, where members are kicked out if deemed too ugly. The latest arrival is Luxy, an app for those who don’t want to date anyone who needs to split a bill. Or, to use its own description, ‘Tinder without the poor people.’

Luxy has been deplored for its overt snobbery but it is, in effect, a digital version of what used to be called the London Season. The idea of the rich marrying other rich people is making a comeback — and alongside it there is another trend, as modern as the internet itself. Those at the top are deadly serious about marriage, prepared to invest time and money in it. But among those at the bottom (often, those under a welfare regime which makes couples poorer), marriage rates are steadily falling. A marriage gap is now opening, bringing with it deep implications for social cohesion.

I have just finished filming a documentary for Channel 4 Dispatches entitled How The Rich Get Richer, looking at inequality in all of its dimensions. We commissioned the Centre for Social Justice (on whose advisory board I sit) to trawl crime, school, wealth and deprivation figures for every neighborhood in the country. Many of the problems have changed depressingly little over the years, in spite of the billions spent in Labour’s battle against poverty. But one inequality was growing faster than any other: that of marriage. For those safely in the top tax bracket, (the cohort which Luxy is targeting) nine in ten new parents are married. For those on minimum wage or less, it’s about half.

Marriage used to be the norm for everyone in Britain. When the Dixie Cups’ ‘Chapel of Love’ was in the charts in 1964, some 93 per cent of children were born within marriage — the same as when records started in the 1850s. But that has now changed, utterly. Strip away immigrants (who tend to be more socially conservative) and almost half of British babies are born to unmarried parents. Cohabitation has not proved stable — today, the average British 15-year-old is more likely to have a smartphone in the pocket than a father in the house. But even this fact disguises massive divergence in social class. The Office for National Statistics conducts surveys every three months, asking Brits about every aspect of their lives and dividing workers into seven social categories. At the top comes ‘higher managerial’ — the likes of company directors, military officers and university lecturers. At the bottom come the ‘routine occupations’ such as cleaners, builders and waiters. Marriage data is not normally published, but supplied upon request.

It shows that there was already a pronounced marriage gap in 2001, when the figures start, with those in the top category 24 per cent more likely to marry than those at the bottom. That figure now stands at 48 per cent.

So a marriage gap that barely existed a generation or two ago has managed to double in the last decade with a minimum of public debate. Somehow marriage, with all the advantages that it confers, is becoming the preserve of the rich.

It’s clear why the rich act as they do. In the documentary I speak to Karen Mooney, who runs a dating agency whose joining fees run up to £10,000. Her clients, she tells me, are thinking so far into the future they plan not just children, but the inheritance they’d like to pass down to those children. The worst thing is that it makes perfect sense. Once, a middle-class earner could afford a family house in a leafy area, and school fees if needed. Now, even the well-paid feel they need to double up their income through marriage before they confront the financial K2 that is family life.

After the death of Prince Albert in 1861, royal parties were cancelled for three years while Queen Victoria mourned. The London Season was shelved. Academics from Pompeu Fabra university in Barcelona recently looked at marriage records to find out what happened next: young debutantes were 80 per cent more likely to marry a commoner. Had the Season been cancelled a decade earlier, they calculate, newcomers would have been 30 per cent more likely to enter the English elite.

Matchmaking technology may have evolved since then, but the underlying principle has not: if the rich are more likely to marry the rich, the inequality problem becomes worse.

This inequality of marriage ought to concern the left. There has been far more family breakdown over the last four decades, but it’s the poorest who are being most affected. There are no absolutes in this argument — successful families do come in all shapes and sizes — but figures do show a broad trend. Fewer than one in ten married parents have split by the time a child is five, but a third of unmarried parents do so. As Tony Blair said, ‘A strong society cannot be morally neutral about the family.’

David Cameron agreed, once. He said he was passionately in favour of marriage, and spoke up for it at the last general election. But the idea of a tax break unnerved the more socially liberal George Osborne, who has refused to implement it until the last four weeks of a five-year parliament. Nick Clegg is hotly against the whole idea, saying politicians should not make ‘moral judgments’. So even talking about the family, far less promoting it, is difficult for a reviled political class terrified of being seen to lecture voters.

The result is a creeping social segregation which is not being discussed, far less addressed. The marriage agenda has fallen foul of a new cross-party consensus: that the toughest questions in politics are, nowadays, best avoided.