Gay best friends are one thing, however; gay marriage is another. While the legal distinction between civil partnership and gay marriage may be subtle, the implications of the latter are incomparably greater. Marriage is not only a secular but a religious institution, which Christians, Jews and most other faiths define as a bond between a man and a woman. The main exception, Islam, prohibits homosexuality much more strictly than the rest, while permitting a man to marry up to four wives (and occasionally more). If legalising gay marriage renders a conflict between church and state likely — at any rate if churches are forced to solemnise such weddings — it renders a confrontation between state and mosque well-nigh inevitable. We know that polygamy is happening on a large scale because multiple "wives" already claim benefits and are entitled to do so, even though bigamy is still a criminal offence. Once gay marriage is legal, Muslims will demand parity. 

Women may be ambivalent about gay marriage, but the vast majority does not regard polygamy, let alone the other aspects of sharia that relate to women, as acceptable. The institution of marriage matters more to women than to men, because it is designed to protect them and their children during their most vulnerable years. Successive governments and social changes have undermined marriage in countless ways, but it remains the indispensable source of security for women who wish to raise a family. Women are protective of their hard-won rights and do not take kindly to any threat to their equality before the law. Gay marriage does not directly threaten that equality — indeed it is depicted as rectifying an inequality — but it may also be seen as a Trojan horse. Once the secular definition of marriage as monogamous is extended to include same-sex couples, what is to prevent the criminalisation of core principles of Judaeo-Christian morality? If monogamy is up for grabs, how can we be confident that the law will continue to exclude from marriageability not only polygamous relationships but others hitherto prohibited, such as those with children? Women are unlikely to welcome yet more confusion and moral relativism in a domain for which they have traditionally taken primary responsibility. They may feel that the government would do better to leave well alone.

In Manchester, the Prime Minister offered one overriding justification for gay marriage: commitment. "Conservatives believe in the ties that bind us, that society is stronger when we make vows to each other and support each other. So I don't support gay marriage despite being a Conservative. I support gay marriage because I'm a Conservative." This is indeed a persuasive argument to the 21st-century mind, especially when allied to Mill's principle that "liberty consists in doing what one desires". But Mill also wrote: "The liberty of the individual must be thus far limited; he must not make himself a nuisance to other people."  There is nothing to stop gay people making vows to and supporting each other as things stand; indeed, civil partnership, an institution less than a decade old, was created exclusively for that purpose. (I say exclusively, because it deliberately excluded other, less organised groups who, like homosexuals, live together but cannot marry — an enduring grievance.) What makes marriage unique is not commitment, but procreation. Cameron made no mention of this, but of course it has for some time now been illegal for adoption agencies to exclude same-sex couples; the Catholic Church was thereby forced out of the adoption business. It is no longer rare for gay couples to have their own children, too: Sir Elton John became a father last year at the age of 63 with the help of a surrogate mother. Lesbians, in particular, have proved themselves to be able to create loving families in which children can thrive. Marriage might seem to be the best way of protecting such families; indeed, Cameron clearly believes this.