It's possible to make a case against gay marriage that does not rely on fear or loathing of gay people. That's what was done when civil partnerships were brought in; and there is a surprisingly long tradition of Christians doing it. In 1996, for instance, Jim Thompson, then the bishop of Bath and Wells, published a book calling for what were in essence religiously sanctioned civil partnerships: "I am in favour of strengthening the social support for gay people to have sustained, faithful and loving relationships by legal agreement and by the prayerful support of the church."

He said then that: "One of the things that helps people towards fidelity in life is proper recognition by society. I don't believe that recognition can be marriage; but there ought to be a recognition of jointly held property in order that people will have things that bind them together."

I don't think it's breaking a confidence now to say that Thompson had told me privately a few years before that his policy was to ordain gay men only when they were in stable relationships.

Incidentally, in those days conservatives were defending marriage on the grounds that it was unnatural: the Rev David Holloway, one of the prime movers of the evangelical campaign against gay clergy, said then that, "Marriage as it has developed is not biologically natural – to keep a father in a committed relationship you need a whole lot of other constraints. The gay issue is the motor for the whole programme of destabilising the sexual culture, and the effects of that on children are disastrous."

This seems to me more honest than the present line, largely taken over from the Vatican, that marriage (understood as monogamous and heterosexual) is something that has been around since the beginning of humanity. But it's less politically persuasive. No one enjoys to be told they need protecting from their own worse instincts; and in a democracy the people get what they enjoy, so long as it doesn't matter.

Yet the argument for civil partnerships, as against gay marriage, seems now to be lost. It hasn't been won by the supporters of gay marriage. It has been lost by the nastiness of the opponents.

When Chris Sugden and Philip Giddings of Anglican Mainstream released their letter to the prime minister last week they cannot have understood just how foul-spirited and pharisaical it makes them appear. They have been taken seriously for so long within the power structures of the Church of England that they have quite lost touch with the sanity of the outside world. They founded their pressure group to oppose the appointment of a celibate gay man as a bishop. Yet they claim in their letter that "those who experience the attraction" – they won't talk about "love" – "have always been fully welcomed".

Condescending and pompous to the end – they finish with the assurance to the prime minister of their continued prayers – this letter discredits all opposition to gay marriage. It's obvious that what they really want is for gay people to feel ashamed and to exist on sufferance. The only thing tending to acquit them of a rather unpleasant prejudice is that their smug condescension isn't only directed at homosexuals. Evangelicals of that sort want everyone who's not like them to feel ashamed of their existence. "We are all sinners", they say, but they think they know they and their friends are saved.

Catholic bishops, too, suffer a terrible disconnect from the ordinary moral sense of the world outside. When Philip Tartaglia's claim that a Scots MP (and former Catholic priest) who died of pancreatitis at the age of 44 did so as a result of being gay surfaced to general outrage last week, few people noticed that he was speaking at a conference on religious freedom.

"I can say with a concerned and fearful realism that the loss of religious freedom is now arguably the most serious threat that the Catholic church and all people of faith in this country are facing," he had said. "Will the Catholic church – and other religious bodies and groups – have the space to adhere to, express and teach their beliefs in the public square? Or will these basic elements of religious freedom be denied, driving the Church and other religious bodies to the margins of society, if not actually underground?"

What's crazy about this "concerned and fearful realism" is that he gives every appearance of believing his own propaganda. He confuses losing an argument with losing the right to argue. There are actually genuine issues of religious freedom and toleration raised by some recent administrative decisions against opponents of gay marriage. But they have arisen because the argument about equality is already lost.

The argument about civil partnerships and fairness can't convincingly be put by people who have been unfair whenever they thought they could get away with it.