British Prime Minister David Cameron has announced that he intends to introduce a tax break for married couples, fulfilling a long-standing promise to recognize marriage in the UK’s tax system (unlike in the U.S., where a married couple can pool their income and deductions, Britain currently treats individuals largely separately for tax purposes). The measure, which is aimed at low- and middle-income earners, would leave a typical couple around $330 a year better off.

It’s not a huge amount of money — and some Conservative politicians, religious leaders and pro-marriage groups say it’s too little to make a difference. However the announcement is as much a statement of Cameron’s personal support for marriage as it is about the money, and is designed to send a message that the state values the institution (Cameron has defended his support for gay marriage in part by claiming it will strengthen marriage as a whole). Along with reforms to education and the welfare system, the promotion of marriage is part of the government’s effort to address the problem of family breakdown and its attendant social problems, which are reckoned to cost the UK some $75 billion a year.

Reaction to the announcement from the left has been predictably hostile. Labor politicians have condemned the plan as “insulting” to single parents (a designated victim group) and couples who choose not to marry. Harriet Harman, the party’s senior token women and a dour, knee-jerk feminist, called it “stigmatizing and moralizing” and “Victorian finger-wagging.” The Tories’ coalition partners, the center-left Liberal Democrats, have been equally critical, with party leader Nick Clegg accusing Cameron of trying to “turn back the clock to the 1950s.”

Why so much outrage over such a modest gesture? The tax system has long discriminated against married couples, particularly if one parent stays at home to raise children, and in favor of couples who separate. Meanwhile, more generous tax breaks have long been the norm in countries including France and Germany, where marriage rates are higher.

No one is suggesting that either single parents or cohabiting couples can’t make good parents, or that every marriage is successful. However, studies have repeatedly shown that married couples are more likely to stay together than cohabiting ones; and that marriage provides the most favorable conditions for raising children who will do well at school and have better job prospects, and avoid alcoholism, drug addiction and other problems. Stable marriages and strong families are also powerful engines for lifting people out of poverty and increasing social mobility.