Apparently, my history teacher was wrong and Voltaire never actually said: "I despise what you say but I will defend to the death your right to say it." As quotations go, it's only GCSE clever – an interesting juxtaposition for young teens, a notch above "'Assume' makes an 'ass' out of 'u' and 'me'." If you're still quoting it at A-level, that's OK but you won't win any prizes. In an undergraduate essay, it should get no more credit than "Too many cooks spoil the broth" or "At the start of the war, few of the combatants knew how it was going to end."

While it's an important sentiment, it should also, in a mature, free country, be an obvious one. It's not complicated – it's a truism, not a paradox. Having accepted it as a premise of our society, we should be talking about more contentious things – whether the pause that electronic equipment now makes between our pressing a button and its obeying is the first step towards Matrix-style insurrection, or how many episodes of Top Gear you can enjoy before your soul is forfeit.

So why, when the small bunch of extremists, bigots and opportunists that is Islam4UK announced that they wanted to stage a demonstration in Wootton Bassett, did the home secretary say he would support attempts to ban it? They're a horrible organisation – an offshoot of the al-Muhajiroun movement that is opposed to the British state and arranged, on the first anniversary of 9/11, a conference entitled "A Towering Day in History" (although a pun is always welcome). I can understand completely why Alan Johnson despises what they say, and I agree, but I'm not certain that outlawing their demonstration qualifies as defending to the death their right to say it.

I hate the idea of the demonstration going ahead. If Islam4UK were sincere in their desire to point out that Muslim civilians have died in Afghanistan as well as western troops and to express their view that it's an unjust war, then they wouldn't have chosen Wootton Bassett as the location. They've chosen that new mecca for Britain's expression of military grief because they wish to defile our holy places. As British right-wingers always say in response to Islamist protests, in most Islamic countries the equivalent wouldn't be permitted. That's both true and a direct index of the liberal democracies' moral superiority. When they allow a gay pride march through the real Mecca, Saudi Arabia may be a country worth visiting.

The thing about freedom of speech is that people are allowed to say offensive, indefensible things; that we needn't fear that because we're sure that wiser counsels are more likely to convince. "Let the idiots and bullies speak openly and they will be revealed for what they are!" is the idea. It's a brilliant one and, in confident, educated societies, it almost always works – certainly much more often than any of the alternatives. Why has Alan Johnson lost confidence in this principle? Why have the 700,000 signatories of a Facebook petition calling for the event to be banned?

I know there are circumstances in which freedom of speech is rightly limited – I'm not arguing for a repeal of all libel or incitement to hatred laws. But it's difficult to see how this demonstration would incite hatred of anyone other than the demonstrators. Public safety can also be an issue. I understand that the police couldn't let the protest go ahead without a reasonable expectation that it wouldn't become violent. But if it is banned, let us be 100% sure, let our consciences be absolutely clear, that public safety was the reason, not the excuse.

One of the accusations fairly levelled at Islam4UK (incidentally, having now typed their name three times, I almost feel like it's the most offensive thing about them) is that they're cynically trying to garner publicity. In fact, their leader, Anjem Choudary, admits it, saying: "It is a publicity stunt, you can call it that, to create awareness." They've announced headline-grabbing marches in the past and then cancelled them. Our response, according to the Muslim Council of Britain, should be "to ensure we do not grant them the oxygen of publicity".

Whoops. I may have given them half a lungful. But this is a situation in which the Islamists and the rabble-rousing, right-wing press are mutually oxygenating. Islam4UK makes a big splash, probably without even having to go to Wiltshire, and the newspapers get to show outrage and assemble a bunch of scandalised quotes from politicians who should have better things to do. Middle England is confirmed in its prejudice that there's nothing to which some Muslims won't stoop, and those hostile to the British regime are confirmed in theirs that it smacks of repression.

This wouldn't happen if we remembered our misattributed GCSE Voltaire. If it were unthinkable that the demonstration would be banned, other than for reasons of prevention of violence, then the reaction to its being planned would be altogether different. We'd accept that, in a free society, people don't always exercise their freedoms nicely. And, with less free publicity on offer, the chances of such marches being mooted in the first place would be reduced.

The other great boon of that state of affairs – still nominally this state of affairs, let's not forget – is that we can reply. We don't have to show the slightest respect for other people's views – just for their right to hold them. Respect, after all, must be earned. It's only freedom of speech that's a right. When someone says something which you find stupid or offensive, you can say something back. You can tell them to fuck off. They don't have to, but they've still been told.

Maybe that's not your idea of utopia – millions of people screaming: "Fuck off" at each other – but it beats banning it, making an opinion against the law. When Jan Moir wrote her unpleasant article about Stephen Gately's death last October, among the perfectly appropriate calls for her to fuck off were some for her to be prosecuted for inciting homophobia. That's nonsense. Her offensive opinion was her right. To die in defence of that would be honourable, if not entirely natural.