Religious extremists tend not to have a great sense of humour. So I don't imagine that the people who wanted to plaster anti-gay ads on the side of London buses were consciously trying to echo a well-known Monty Python sketch. But some of the phrases used in the campaign – "ex-gay" and "post-gay" – reminded me irresistibly of John Cleese ranting about his "ex-parrot". This is what happens, I'm afraid, when humourless idiots try to be clever at someone else's expense.

Indeed, I was sorry when the Mayor of London, Boris Johnson, decided that Londoners wouldn't be able to see these risible ads, due to appear on two-dozen buses in central London from tomorrow. Johnson is running for re-election next month and he was never going to risk alienating the capital's gay vote by nodding through an undeniably offensive slogan. But he now faces a possible legal challenge from one of the Christian groups behind the ads.

They're complaining about censorship and portraying themselves as victims, which is one of several reasons why I believe it was a mistake to pull the bus campaign. It's almost always better to challenge bad ideas than to ban them, which runs the risk of appearing to suggest that they're dangerous and radical. Secular and equality campaigners are wittier and more sophisticated than the anti-gay lobby; think of the fun we could all have had with post-this, that and the other jokes.

In any case, the Christian attempt to subvert Stonewall's slogan in support of gay marriage – "Some people are gay. Get over it!" – backfired spectacularly. If people are "ex-gay, post-gay and proud", why do they need to "get over it"? Is this about "not gay" pride or isn't it?

But there's another reason why I believe the campaign should have been allowed to go ahead. Over the past few months, we've heard a great deal about "aggressive secularism" from Eric Pickles, Baroness Warsi and church leaders. What they're complaining about isn't aggressive – it's the application of principles of equality that don't give special privileges to believers. But aggressive Christianity is on the rise, and a nasty campaign against gay marriage is the latest attempt to halt advances in human rights that benefit women, homosexual people and secularists.

In that sense, the anti-gay ads have already performed a useful service, challenging the notion that the modern church is packed with sunny-tempered coves whose slogan is "live and let live". The Christian groups associated with the campaign are homophobic and irrational; one of them, Anglican Mainstream, has supporters who compare homosexuality to alcoholism. Its website talks creepily about helping gay people to realise their "heterosexual potential", and a letter calls for professional help for people who want to "resolve unwanted same-sex attractions". Its signatories include Lord Carey, the former Archbishop of Canterbury, and Michael Nazir-Ali and Michael Scott-Joynt, former bishops of Rochester and Winchester respectively.

Twenty years ago, gay people were still trying to get rid of Section 28. Now there are civil partnerships and we're moving towards gay marriage. The religious right is becoming more aggressive about equality and human rights issues – abortion as well as gay marriage – because it's losing the arguments. I mean, who really thinks that "not gay" pride will catch on?

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