An organisation dedicated to increasing discrimination against a long-persecuted minority has just been set up, and – as happens a little too often with such things – received a glowing recommendation from a former Archbishop of Canterbury. Writing in the Daily Mail earlier this week, Lord Carey sang the praises of the Coalition for Marriage. “Don't play politics with marriage”, this organisation declares on its website. “One man + one woman.”

It advances three main arguments: first, "the evidence shows children do best with a married mother and father"; second, "if marriage is redefined once, what is to stop it being redefined to allow polygamy?"; and third, "it's not discriminatory to support traditional marriage". My dear friend Dave Sampson, a lawyer now relocated to San Francisco and scoring victories for the heroic National Center [sic] for Lesbian Rights, has been in touch to explain the stupidity and abhorrence of all three.

The first is a lie. As Dave writes: "Hours of witness testimony in the Prop 8 trial in California showed that same-sex parenting is just as effective as that provided by straight couples." The second is slippery-slope reasoning – which is to say, sloppy reasoning. The third misunderstands discrimination. Civil partnerships would be equal to marriage if given the same respect by society. But "the Coalition for Marriage, Lord Carey and the like", says Dave, are "proof positive that this is not the case".

Lord Carey's intervention exemplifies religious thinking standing athwart history, yelling Stop! Quite why the former Archbishop's knowledge of the Gospels, and love of Jesus, enhances his authority on this subject is unclear to me.

Marriage should be extended to gays because, as the best social institution we have yet created for promoting love between adults, it is wrong that a vast section of humanity be denied it. The dignity and affection granted by marriage should not be determined by whether you are gay. It should be determined by whether you are human.

Much though it might dishearten Lord Carey, the issue here is not religious, but civil. Religious groups have the liberty to endorse gay marriage if they wish – or not, as the case may be. They have this thing at their disposal called freedom of conscience. That matters at least as much as the right of gays to be treated equally under civil law. And anyway, why can't the two co-exist?

Marriage is a precious institution that has rarely been so weak in the rich world. Given it should be cherished, the idea of an Archbishop preserving it for his own is repulsive. Lord Carey's intervention isn't merely the worst of religion. It's theocracy.

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