If you’d told me in 2000 that in thirteen years gay marriage would be made legal – by the Tories – I would have laughed you out of the room. Back then homosexuality was still vaguely taboo almost everywhere, and my own school, where I was sitting A-levels, was not unusual in its culture of brutal homophobia.

But over the next few years society saw a quiet revolution in attitudes: most of us realised we had nothing to fear from gay people, and soon it wasn’t homosexuality that was unacceptable (at least in public) but homophobia. By 2013 the once unthinkable had become common sense and a law allowing gay marriage was passed, with the first couples due at the altar at the end of March 2014. For liberals like me, it was a giddy moment: at last it seemed our leaders were ready to treat us like moral grownups and grant us our basic freedoms.

And then something took place that made me realise how far we had to go – and at the centre of it was the party with “liberal” in its name.

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Here’s what happened. First Maajid Nawaz, a Liberal Democrat candidate for parliament in 2015, tweeted a bland cartoon from the online comic strip Jesus and Mo, declaring, “This cartoon is not offensive, and I am sure God is greater than to be threatened by it.” Nawaz is a Muslim who has written a book about his extremist past, so once upon a time he might have been among the online mob that quickly arrived, determined to feel threatened on God’s behalf. What followed was the usual incitement, abuse and death threats from radical Muslims, who hope to intimidate others into obeying their rule banning depictions of the Prophet Mohammed.

Then came the depressing part. Instead of rallying around a liberal Muslim colleague and defending his right to free speech, a number of Lib Dems disgracefully flirted with support for those threatening to murder him. Nawaz should use his freedom of expression more “responsibly”, pontificated some (so claiming not to be offended by a bland picture is irresponsible, right?). Shafiq Mohammed, a party activist, persuaded 23,000 people to sign a petition to have Nawaz removed as a Lib Dem parliamentary candidate. And Gordon Birtwistle, an MP, wrote to a constituent to say Nawaz should be deselected and booted out of the party – a sanction even Chris Huhne avoided initially when he was sent to prison.

Even those who stood by Nawaz – like party leader Nick Clegg – tempered their support by intoning gravely about balancing free speech with sensitivity and respect. Which sounds reasonable enough, of course. Except freedom of speech without the freedom to offend is not only a meaningless idea – it’s a dangerous one, and it’s quietly gaining ground. If I can frighten you into silence by claiming your ideas upset me, then your right to free speech becomes purely theoretical, like a right to jump over the moon.

The beautiful thing about liberalism – the supposed political creed of the Lib Dems – is its simplicity. Its most basic idea is that you are free to do or say whatever you like: you can spend all day praying or in S&M orgies, if either tickles your fancy; as long as you don’t cause anyone else any actual harm, you’re good to go.

But in Britain today a coalition of religious bigots and misguided leftwingers is attempting to alter – subtly, but irreversibly – the outer limit of free speech. Instead of allowing speech unless it harms, this group wants to prevent speech that offends. Notice that among all the calls for Maajid Nawaz to be forcibly shut up (from goons on twitter to Lib Dem apparatchiks in Westminster) nobody even tried to pretend the tweet had caused any real harm to anyone. What they claimed was that it had offended them by failing to accord their crazed dogma the proper respect.

This language of “offence” can suddenly be heard everywhere, even among so-called liberals. Under the new unspoken rules of liberal discourse, you can win a debate not by producing a killer argument or fact, but by claiming you're offended – ideally by somehow invoking minority or victim status.

Nothing offends the anti-offence brigade as much as mockery of other people’s beliefs. But laughter is our best weapon against absurdity of any kind: give Armando Iannuci a satirical news show on prime time North Korean TV, and – putting aside the fact that his life would quickly be worth as much as a North Korean banknote in, well, North Korea – I doubt the authorities would survive past the middle of season two.

The day we lose our right to openly mock or offend, we can kiss our right to free speech goodbye. And without free speech, all our other freedoms are under threat, because we won’t know how to protest when they come under attack from the powerful.

Want an example of the kind of thing that’s becoming too offensive to say? Are you sure? Okay: all the evidence shows that, although in recent years homophobia has receded dramatically in the general population, it is still rife in Muslim communities . We can begin to rectify this: by legally requiring Islamic state schools – along with all others – to promote equality for LGBT people. And ordinary people can help by supporting groups which speak out for gay Muslims, like Imaan.

If this offends you, I’m sorry. I understand why many of my fellow liberals and leftists prefer to avoid such issues: because we know that Britain’s Muslims are already feared and distrusted, and we are wary of giving their enemies another excuse for their hate. I am not saying, of course, that there is anything intrinsically wrong with Muslim people or Islamic culture. Nor am I saying that other communities don’t have problems: the Jimmy Savile scandal has exposed the (predominantly white, upper-middle-class) media establishment’s deep-rooted culture of impunity for powerful men who harass and attack women and even children.

But we didn’t shirk from exposing child abuse by priests simply because many Catholics are among the worst-off in Britain. And pretending Islamic homophobia does not exist simply condemns a generation of gay Muslim kids to misery and self-hatred. What's more, it's subtly racist: as though Muslims mustn't be held to the same standards as others. True liberals should not hesitate to condemn both prejudice against Muslims (which is real and pervasive) and prejudice by Muslims.

There’s a clear difference between speech that merely offends and speech that truly harms. I am offended every time I hear someone say that wealth and poverty reflect innate differences of ability, that feminism has gone too far, or that homosexuality is a disease that can be “cured”. But I don’t want those views banned. If someone expresses an opinion you don’t like, fight back – with reason, facts, and evidence. We should all be as big as Maajid Nawaz and his God, and feel perfectly unthreatened by sticks and stones and tweets.