If ever you needed reminding that Britain and the United States are divided by a common language, consider the fact that a Conservative-led government in London now endorses gay marriage. As contenders for the Republican Party's presidential nomination insist that same-sex marriages must destroy marriage, their conservative counterparts in Britain argue that politics and morality alike require the government to commit to legalizing gay marriages.

Consequently David Cameron's government has announced a consultation on introducing legislation legalizing same-sex marriages before the next general election, scheduled for 2015. Crucially, the consultation is not a matter of "whether" to welcome gay couples into the marital fold but "how" to do so. It is a welcome development.

Supporting gay marriage, though too much for the crustier breed of conservative, is nevertheless a sound Tory principle: commitment cannot be determined by sexuality, and there's no reason to deny or refuse to respect the sincerity and value of same-sex relationships. A political party that believes in the value of marriage should not be restricting its benefits to heterosexual couples.

So this development satisfies liberal aspirations while also, happily, demonstrating the value of timeless Tory principles. Society changes, and the proper sort of Tory, however much the past attracts him, appreciates that conservatives must also change if they're to protect the values they hold most dear and remain relevant in a much-changed world.

Cameron is keeping a promise he made when he became Tory leader. As long ago as 2006, he declared, "There's something special about marriage. It's not about religion. It's not about morality. It's about commitment. When you stand up there, in front of your friends and your family, in front of the world, whether it's in a church or anywhere else, what you're doing really means something. Pledging yourself to another means doing something brave and important. You are making a commitment. You are publicly saying: it's not just about 'me, me, me' anymore. It is about we: together, the two of us, through thick and thin. That really matters. And by the way, it means something whether you're a man and a woman, a woman and a woman, or a man and another man."

It used to be said that the Church of England was the Tory party at prayer. Doubtless it remains the case that religious Britons are, all other matters being equal, more likely to vote Conservative. But as far as civil marriage is concerned, there is no need to pander to the objections of a faithful minority. Indeed, Britain is a largely secular society these days and, at least as far as same-sex relationships are concerned, a much more civilized place than was the case in years gone by.

Even so, it is striking how quickly society has changed. Homosexuality was decriminalized only in 1967; less than 50 years later, a Conservative-led government is preparing to legalize homosexual marriages. By doing so, it accepts the argument that civil partnerships—introduced by Tony Blair's government—are an unsatisfactory compromise that, actually, do much more to undermine the institution of marriage than does welcoming gay couples into the marital congregation.

Moreover, civil partnerships—a step forward as they may once have been—still fall short of full equality. Eliminating this discrepancy—it seems likely that Cameron's commission will recommend scrapping civil partnerships—removes the stigma, however minor it may have seemed to some, of second-class status.

The churches may disagree. But there's no reason to grant them a veto over the civil definition of marriage. It is unlikely churches will be compelled to recognize gay marriages any more than a Roman Catholic priest must be cheerfully expected to officiate at a Protestant marriage ceremony. Though often conflated, the civil and religious stamps of approval are different, and the latter need not be expected for every brand of civil marriage.

Cameron's office briefed reporters that the government's decision to upgrade same-sex relationships from civil-partnership status to full-scale, no-holds-barred, no-questions-asked marriage was backed, and indeed prompted, by the prime minister's enthusiasm for the issue.

There is a political benefit to be gained from this, too: it reinforces Cameron's own brand of "compassionate conservatism" and underlines the fact that, no matter what anyone might think about his economic policy, he has changed the Conservative party for real and for good.

More than anything else, however, it recognizes that society has changed and, correspondingly, so must sensible conservatives. By doing so, they may make a virtue out of necessity and inevitability while reaffirming the essential importance of marriage. Low politics and high principle meet in a convenient but passionate marriage of their own. Eventually, perhaps, even the Republican Party in the United States will appreciate this.