Prosecutions for the possession of the filthiest pornography confirm foreigners’ suspicions that the British care more for animals than people. Between 2008 and 2011, the English and Welsh authorities charged 1,922 men for having images of bestiality about their person. By contrast, they brought only 310 prosecutions for possession of pornography that simulated serious or life-threatening injury on members of the human species. For necrophilia, they managed four. And there have been no prosecutions at all of those who downloaded images of rape.

The United Kingdom’s Criminal Justice and Immigration Act of 2008 defines “extreme pornography.” It holds that rape is not extreme enough to be worthy of the law’s notice. The authorities punish those who find pleasure in the abuse of sheep but not in the abuse of women. Censorship rarely rests on coherent principles.

Perhaps because of an administrative error, however, the British government’s refusal to criminalize simulations of rape featuring consenting adult actors has a logic behind it. Liberals stick, or ought to stick, to variations on philosopher John Stuart Mill’s “harm principle”: adults can do, read and watch what they want as long as they do not directly harm others. They are free to express radical views the mainstream deplores, unless they urge readers to murder a banker or plant a bomb. They are free to be a racist or a homophobe. But they are not free to bawl racist or homophobic slogans to mobs by a mosque or gay bar.

In the case of simulations of rape, no one can prove that they provoke actual rapes. In 2005, the U.K.’s Home Office and Scottish Executive (the executive branch of the devolved government of Scotland) said they had no evidence on “the likely long term impact of this kind of material on individuals.”

When rape crisis centers, the deputy children’s commissioner (whose duty it is to promote awareness of the views and interests of all children in England) and scores of academics and women’s rights campaigners told Prime Minister, David Cameron, they wanted a new law, the Ministry of Justice, the government department responsible for providing access to justice and for areas of constitutional policy, dismissed their open letter. “We have no evidence to show that the creation of staged rape images causes harm,” it said.

Dr. Clarissa Smith of Sunderland University and a co-editor of Porn Studies — a learned journal, despite its name — says experiments that link pornography to the abuse of women are riddled with uncertainty. Sex offenders are more than eager to tell researchers that pornography turned them into criminals. They can shift the blame and refuse to accept responsibility. Psychological tests on the effects of sexual images on male aggressiveness are little better. Put crudely — and there is no way of avoiding crudeness here — if a psychologist shows young men pornographic videos and then makes them answer questions instead of allowing them to go home to masturbate, those young men are likely to turn aggressive.

Many of those who want to ban surprise you by half agreeing. Professors Clare McGlynn and Erika Rackley of Durham University signed the open letter to Cameron earlier this month. But they do not maintain “that the viewer of pornographic images of rape will necessarily go on to commit sexual offences.” Rather, they and many others believe that the absence of hard evidence does not matter, and I understand why.

Imagine if a time machine could transport a newspaper reader from 1973 to 2013. He or she might be pleasantly surprised at how racism had declined. They would be astonished at the triumph of homosexual emancipation. As for women? They would notice advances in workplace, employment and divorce law, but see, too, that misogyny remains acceptable.

It’s not only the pay gap and the glass ceiling. Television stations drive out female presenters for growing old but let their men turn grey. A racist remark is a sackable offense in politics: a misogynist insult is light-hearted banter. A portion of the left cannot see anything wrong with allying with misogynist Islamists. A portion of the right cannot see anything wrong with wanting women to stay in the kitchen.

To use the available cliches, sexism is neither a “red line” nor a “deal breaker.” All kinds of people can indulge in it without fear of the consequences. The wonder of our day is not that parts of the world are experiencing a feminist revival but that it has taken so long to stir.

Dominating all else there is the explosion in pornography. Works you could not have bought in the dingiest basement in 1973 are now online and freely and legally available. You may not be able to prove a causal link between pornography and violence. But those who are pressing Cameron — and pressing at an open door, I suspect — worry about a culture in which sexual violence is condoned and seen as entertainment; where “basically porn is everywhere,” in the words of the children’s commissioner. They then turn their gaze to a legal system that can hardly bring itself to convict rapists. When websites advertise with the slogan “nothing is better than seeing these good-looking sluts getting raped,” you may well feel in your gut that their viewers should be arrested.

Never confuse your gut with your brain. The old principle that consenting adults are free to watch what they want is worth defending, not least because it is not as permissive of porn as it seems. It criminalizes child pornography, naturally, because children cannot give informed consent. It could just as easily order Internet service providers to require that their subscribers prove they were adults before allowing them to watch “adult” entertainment. If a dirty old man were to hand out hardcore material at school gates, the police would arrest him. When media companies facilitate the delivery of hardcore porn to the mobiles and tablets of children who cannot make informed decisions, the law lets them be.

Think a hundred times, however, before you go further and tamper with the consensual behavior of adults. Everyone who wants wide-ranging censorship prefers vague claims about ill-defined threats rather than precise information. Intelligence services want to ban all revelations about their work to preserve their mystique and hide their crimes. Religious people want to punish all cases of blasphemy to protect their jealous god or gods. Homophobes argue against homosexual equality because it poses some hazy threat to heterosexual marriage.

If the protesters can find a link between the mysterious world of consensual sexual fantasy and crime, they would be right to ban. But, if they charge ahead and legislate without even bothering to explain the connections between image and reality, word and deed, they will give up more than they know.

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