Philip Hensher: Against gay marriage? Too late, it's already here

I don’t know a single person in a civil partnership who doesn’t refer to themselves as being married

Opponents of gay marriage have, thus far, made much of the idea that if the Government gives legal recognition to same-sex marriages, then there is no reason why it should not go on to recognise polygamous unions. Defenders of gay marriage say, on the other hand, that the "slippery slope" argument, by which you will find yourself recognising a union between a man and two women before much longer, is nonsense.

Unfortunately, they are both wrong on this point. Polygamous marriages are already a fact in law. It is difficult, but not impossible, for more than one polygamously married spouse to enter the UK from a culture which recognises such marriages – they can't enter as spouses, but they perfectly well could through some other method. It is difficult to be sure of the exact numbers, but there may be 1,000 or fewer legally recognised polygamous marriages in the UK.

The suggestion that anybody owns marriage, and can say what it is, has been, and always shall be, dissolves on the slightest investigation like rice-paper confetti in rain. The Anglican Church spent most of the 19th century orchestrating opposition to the outrageous suggestion that if a man's wife died, he should be permitted to marry her sister – the Deceased Wife's Sister Bill was passed only in 1907. Hindu widows weren't permitted to remarry until 1856. Churches nowadays seem to be able to cope with those former assaults on the sanctity of marriage. The marriage of convenience, the dynastic marriage, the mariage blanc – all belong to particular historical circumstances which arose and which might disappear again.

My own husband's great-grandfather married polygamously – no one else in the family ever did so again, and the question would hardly arise now. When I hear permanent and universal definitions of marriage sonorously uttered on Newsnight, I think of my great-grandfather-in-law, who would hardly recognise any of them. Do they mean that his wasn't a marriage, despite what he, his wives and his whole culture thought at the time?

More to the point, a church should not dictate the nature of secular marriages at all – whether between two people of the same or of different sexes. We choose, these days, whether to let a religious authority have any say over our lives in the way that we do not choose the force of law. Why should the Roman Catholic Church, the Anglican Church, or any other church have the slightest say over who can or cannot get married, without that person's consent? Let's put it like this: if you don't like gay marriage, fine. Don't get gay married.

In recent days, opponents of the gay marriage proposal have argued that we need to have a debate about the nature of marriage. Clearly, we are having this debate, and have been having it for centuries. On this point, the debate is concluded, and the opponents of gay marriage have lost. They have also said, in chorus, that they have nothing against gay people: indeed, they think gay people are splendid, and that civil partnerships ought to be enough for them.

This claim would be more convincing if these people could indicate a single historic occasion when they have supported the award of a small fragment of civil liberties to gay people. From the Wolfenden report through the 1967 Act through the right to serve in the military to the creation of civil partnerships, churches and politicians have argued exactly the same thing: gay people don't deserve it, and they will destroy the civilisation of "normal" people. They were wrong then, and they are wrong now: not only wrong, but, in many cases, dissimulating their real attitude.

The Catholic Archbishop of Westminster, Vincent Nichols, has said he hopes that the debate can be "measured and reasonable". It would be flippant, but accurate, to say that it probably will be, so long as he and his church stay out of it. Measured? His colleague in Scotland, Cardinal Keith O'Brien, kicked off his contribution by comparing the introduction of gay marriage to the forcible reintroduction of slavery, and calling gay relationships "grotesque". Reasonable? His church's claim that marriage is valid only if there is a chance to produce children will have sounded shockingly medieval, not just to gay couples, but to heterosexuals wanting to marry later in life, or where medical factors rule the possibility of children out.

Like it or not, the definition of marriage has changed, in the most literal, linguistic way. I don't know a single person in a civil partnership who doesn't refer to themselves as being married and having a husband or wife. In the popular mind, marriage has extended itself since the introduction of civil partnerships in 2004. There may be some muddle along the way, but no one, in my experience, ever says anything but "When did you get married?". Against the might of the shared attitude, and, even more, against common usage in the English language, church and politicians are perfectly powerless. The same-sex marriage, in almost everybody's mind, has already happened.

And that is as it should be. Marriage is a wonderful institution. It brings great happiness to people's lives – not just for themselves, but as a result of seeing happiness in others. It takes many shapes and many forms, not all of which would have been conceivable 100 or even 50 years ago. What the opponents of same-sex marriage have missed is the shift in attitude which means that people understand that good, moral, committed lives can be lived in any number of ways; that it is up to us to discover the best path for us, and for politicians to enable our lives' progress, not to stand in their way. In 100 years' time, this will look exactly as controversial as the Hindu Widows' Remarriage Act of 1856, the Deceased Wife's Sister Act, 1907.