The madness of the Tory party breaks new bounds with the gay marriage revolt. How did it come to this? Once proud to be the natural party of government, they have not only lost their marbles but they are throwing them at each other. Factions gather around the most improbable leadership contenders. Turn right! they cry, though their despised leader is the most rightwing prime minister of modern times. They are the only ones deceived by his moderate demeanour. Political failure usually breeds frenzy, as we gaze in on a party eating itself. What hope for the Tory future where the young "moderns" of the 2010 intake are even greater market fanatics?

Everything about the gay marriage debate makes the party look lost to common sense. Here we are in the depths of the worst depression, the weakest of any large economy, with investment 20% below its peak. Most of the population can feel their living standards fall further each year, yet the government is not for turning. The prime minister turns his attention to Timbuktu while shunting crucial economic decisions – from a new airport to our European referendum fate – into the blue yonder. The state shrinks as cuts accelerate leaving more long-term young unemployed than ever. The screw tightens on benefits, while a ratchet of bankrupt hospitals and rushed closures will stir local protests. All this is more than enough to cause tumult in the main party of government. But no. They're pretty content with all that, many of them clamouring for more, and for deeper, faster cuts.

What reasonable observer would expect gay marriage to seize their passions instead? US-style culture wars have broken out – but only within the ranks of the Tory party. Deep divides exist on many social issues, but usually the other side is at least comprehensible to the majority. We can understand why a minority of people are profoundly upset by abortion, but this arcane marriage dispute is beyond the ordinary comprehension of anyone not guided by the Bible. The anti-gay brigade built their barricades but failed against civil partnership, which gave gay couples equal rights. Although marriage is no more than a mystical word, adding no new rights, fighting over that word lets homophobes again vent abhorrence at the modern world and all its filth.

Instead of seizing the chance for a new beginning, the ascending archbishop assumes all his church's mysterious sexual hang-ups. Though the C of E and Catholics will be legally banned from conducting weddings, he pretends to fear his priests will be dragged by the European courts to the altar to conduct same-sex marriages against their deepest disgust. Who wants to be married by a gay-hating priest anyway?

Daily Telegraph columns claim to fear that fathers and sons might marry to avoid inheritance tax: in eight years no such incestuous civil partnership has emerged. How, they ask, can gay people divorce for non-consummation without defining their dirty practices in law? (This is only used for Catholic annulments anyway.) Oh me, this is the slippery slope to incest, bestiality and polygamy. What's to stop three people marrying or wedding a flock of sheep? All this has been written – proving only that gay marriage is indeed a slippery slope for the sanity of its opponents.

David Cameron will use the balm of marriage to placate Tory rebels later, promising a marriage tax-break designed to bribe co-habitees to the altar with £150 a year. Iain Duncan Smith last week said poverty is caused not by lack of money but lack of a wedding. Mention marriage, and this party vanishes away to the wilder shores of delusion.

As to the Bible, why the cherry-picking? Moses brought detailed rules down from the mountain. In Leviticus and Deuteronomy, God banned homosexuality: "you shall not lie with a male as one lies with a female: it is an abomination". So why don't Christians observe his other rules – banning haircuts, tattoos, eating pigs and shellfish or wearing a garment of two kinds of material mixed together? As for transgender people, "a man whose testicles are crushed or whose penis is cut off may never join the assembly of God". In the same list of rules, if a wife goes to the aid of her husband in a fight by seizing his foe's testicles "then you shall cut off her hand". From those prehistoric times comes that atavistic distaste for homosexuality, wrapped in religion. But that war was lost when gay sex was legalised, and now civil partnership confers equal inheritance rights. In parliament today the old guard rallies for one last stand, but the fight is only over a word.

I admit I started out puzzled by such strong passions on the gay side too. Why on earth do gay people want to be married anyway? Civil partnership is a very fine thing. My dearest old friends, together since the dark days when gay sex was illegal, were civilly partnered a day after the law permitted it. In a moving ceremony, free of all the freighted iconography of "husband" and "wife", here was a pledge of civility between two equal people. Many heterosexuals would prefer that as well. Why be married if you can be civil instead?

But some gay people do want to marry. Why? Those denied the right to a totemic social emblem on grounds of their sexual identity will always feel outraged. Banning gay people from using the word "marriage" suggests they are less worthy. I have no secret hankering to be a bishop, but I feel all women are diminished by being barred. I feel the same about the sight of women in full burqas: social pressure that hides them from sight diminishes us all. Perversely, the anti- campaign has proved exactly why gay people need the right to marriage.

This, warn the old Tory chairmen of the shires, is "shaking the very foundations of the party". If so, they really are done for. Cameron wrongly thought this a clause IV moment to parade a modernised party. Instead, he has revealed them as a nest of bigots. Disunity is electoral poison, and so is a leader losing control of his party. Rebel MPs, like runaway horses, lose their fear of whips. Gay marriage has become a proxy for other undisciplined craziness running through their veins, from hunting to Europe, privatising the NHS to breaking up the BBC, loathing windmills, loving fracking.

Never mind if the public is on the other side, the public is wrong. Never mind if YouGov finds the votes of only 7% might be swayed by gay marriage, for or against. These Tories know they are right, end of story.