There must be a role for Church of England archbishops after they leave office, but anathematising the logical advance of a uniquely progressive social policy should not be one of them. In lambasting the Government's plan to legalise same-sex marriage as "one of the greatest political power-grabs in history", Lord Carey places himself on the wrong side not just of history, but of morality, compassion and reason.

The introduction of civil partnerships was one of the most welcome – and successful – reforms enacted by Tony Blair's Labour government. The social climate turned out to be generally favourable, and for those who took advantage of the new provisions, the change generated more goodwill and personal happiness than almost anything any government in recent years has done. There may even have been a positive economic effect in the flurry of civil partnership ceremonies that followed.

But many homosexuals felt, rightly, that civil partnership, while a big step forward, could be only a transitional measure, and that full equality would be achieved only when gay people could marry, like their "straight" friends and relatives. Some went so far as to reject civil partnerships as a second-class solution that could delay the acceptance of gay marriage, or even remove it from the political agenda altogether.

To the credit of David Cameron and the Coalition, that has not happened. Not only is gay marriage still on the agenda, but Mr Cameron has been consistently staunch in his backing for it. Last year, he went so far as to tell his party conference: "I don't support gay marriage in spite of being a Conservative. I support gay marriage because I am a Conservative." Given that the party conference was hardly going to offer the most receptive audience for such a sentiment, this was a courageous stand to take.

Last autumn the Government announced its intention to legislate before 2015, and a public consultation will be launched next month. This was the cue for Lord Carey's polemic in the Daily Mail, timed to coincide with yesterday's launch of a campaign, the Coalition for Marriage, that will lobby to oppose any change.

Even a cursory reading of the former archbishop's argument, however, exposes the fatal inconsistency of his stance. In penning what is effectively a paean to marriage – as a force for family and social stability – he only reinforces the arguments for extending the privilege to gay couples. For while civil partnerships have been thoroughly positive and confer most spousal legal rights, it is the special status of marriage – as extolled by Lord Carey – which could be described as a last barrier to full equal rights.

Of course, the cause of gay marriage could actually be helped if the opposition is spearheaded by a cleric of such inflexible and clearly outdated views as this former archbishop. And with five European countries, including Belgium and Spain, already recognising same-sex marriage, and others moving in that direction, there is no reason to delay. The Government should take the only enlightened, modern and just course – and stick to its guns.