More than half a million people have now signed the Coalition For Marriage's petition against the government's proposal to permit same-sex marriage. For a campaign that didn't even exist a few months ago, it's an extraordinary achievement. A rival petition supporting the equalisation of the marriage laws has attracted barely a tenth the number of signatures.

There have been signs in recent days that the campaign to prevent what had seemed a fait accompli is beginning to scent victory, at least in the battle for public opinion. The government remains committed to the reform, but in the wake of the coalition parties' poor showing in the local elections there has been a notable lack of enthusiasm for it, especially on the Tory benches. Nadine Dorries spoke for more of her colleagues than usual the other day when she described same sex marriage the other day as a policy "pursued by the metro elite gay activists" that needs to be "put into the same bin" as Lords reform. "Gay marriage" has become a symbol of everything that the Conservative right hates about the coalition and about David Cameron's modernising agenda.

There is, in fact, a persuasive logic to Cameron's conservative case for same-sex marriage. With its history and moral weight, the word "marriage" has a magic that the newly invented status of civil partnership lacks. To invite gay couples to participate in the institution is not only to offer them full acceptance (the "progressive" part). It is also to ask them to embrace the traditional, and conservative, moral obligations of marriage. At the same time, opening marriage to same-sex couples might give it new appeal to younger, liberal-minded heterosexuals currently suspicious of its historic baggage.

But the case has not been well made. It doesn't help that the proposals themselves are illogical and badly thought-through, and would raise more anomalies than they solve. By closing down options, for example refusing to countenance allowing heterosexual couples to enter civil partnerships, the March consultation document missed an opportunity for a genuine national debate on the nature of marriage and the state's role in registering it. Declaring the policy already decided also generated a predictable backlash. The impression of arrogance was not helped by Lynne Featherstone, the Lib Dem minister responsible, offering a "cast-iron guarantee" that the change would be introduced before the next election (a promise she repeated yesterday).

Instead, opponents of changing the law have dominated the discussion. Their greatest success has been in portraying the government's proposals as involving a fundamental redefinition of marriage. Concentrating on the word rather than the substance presents the change as more radical than it actually is (from a practical point of view, the introduction of civil partnerships represented a much greater advance in the state's acceptance of same-sex relationships). It also leads to some fairly reactionary arguments. The Coalition For Marriage states, for example, that marriage "reflects the complementary natures of men and women" -- a position not far removed from a demand that men go out to work while women stay at home looking after the kids. The same suggestion was made in a letter from the Roman Catholic archbishops that was controversially circulated to Catholic schools.

Ironically, such an argument is itself an attempt to redefine marriage, or at least to return to an older definition. Even understood as a relationship between one man and one woman, marriage has changed profoundly during the centuries, from being an institution based on the exchange of property and securing the legitimacy of children to one based on the mutual relationship of the spouses. US Vice President Joe Biden encapsulated it well when he came out in support of same sex marriage at the weekend. It was, he said, "a simple proposition -- who do you love? And will you be loyal to the person you love? That's what all marriages at root are about." This hasn't always been the case.

Rooting marriage in the difference between the sexes rather than their equality, as the Campaign for Marriage does, looks like an attempt to set the clock back. This is why the issue of same sex marriage should not merely be of concern to gay people. Opening marriage to homosexual couples isn't just a recognition that they are now a full part of society. It's also a logical expression of the modern understanding of marriage as a partnership between equals.