The curious thing about common sense is that it is so uncommon. Jacqui Smith, the home secretary, is so short of it that at times she seems uncommonly silly. Last week she unveiled a proposal about prostitutes that is clearly silly, regardless of one’s opinions about the control of prostitution.

Her central idea in these proposals is to make it illegal for anyone to pay for sex with someone who is being controlled for another’s gain. And, crucially, her plan placed the duty on the punter to discover whether the prostitute is controlled by a pimp, a trafficker in human flesh or a drug dealer. Ignorance would be no defence.

Anyone with a tittle of sense would see that this is unworkable and unfair. Yet Smith insists she sees no disadvantage at all, apart perhaps from the necessity of “marketing” the idea to men. I think she is going to have considerable difficulty marketing it to women as well, even to those who disapprove of prostitution in any form.

How could any punter, no matter how well meaning and fearful of the law, find out for sure that the woman of his choice is with him by her choice as well? If she is under duress, she will certainly deny it out of fear of her pimp or of the villains who have bought her into sexual slavery. So will everyone around her. If the punter comes to the wrong conclusion about her he will be prosecuted for a criminal offence, even though he thought he was within the law.

Of course it is wrong to force women into sex against their will in any circumstances. To do so is to break laws that already exist against rape, sexual assault and trafficking. It is also true that there must be some situations that are obviously dubious and that any law-abiding man ought to get out of as fast as possible. If, for example, the girls are very young and speak hardly a word of English, it is a fair bet that something is wrong.

Normally, though, how is a man to tell? I’ve come across a lot of prostitutes, some in the red-light districts of Hong Kong, Bangkok and Luang Prabang in Laos, some in the smarter parts of London’s Mayfair. I once spent the weekend on a boat on the South China Sea with a Playboy Miss April, who distinguished unselfconsciously between “jobs” and “f***-jobs”. I even know of a few women who, between alimony cheques, have occasionally turned a few tricks for men of their social acquaintance, whom they would not normally dream of charging for the privilege. And I know of one woman who charges her lawfully wedded husband for sex. Feminists used to say that marriage itself is prostitution and, to judge from the tabloid newspapers, in some cases it is.

From all this one thing stands out. Prostitutes vary enormously (as do punters) and so do their situations. Some are forced, more or less; others are not. Some are wretched; some seem content. And if there is no way that a man could find out reliably whether a woman is under duress, then to prosecute him for his ignorance is in effect to trump up charges against him. It is unmistakably unfair.

When confronted on Radio 4’s Today programme by this knockdown argument, Smith repeatedly ignored it; she said instead – and irrelevantly - “I’ll tell you what I think is more unfair and that’s that there are women in this country who are effectively held in slavery.” That is a perfect example of what used to be called female argument – irrelevant, emotional and beside the point.

In talking like this she may have revealed her true motives. She would, like Harriet Harman, the minister for women and equality, like to ban prostitution but accepts that the public is “not ready at the moment” for that. However, she knows that voters are strongly opposed to trafficking and sexual coercion. So perhaps she has come up with a ban by the back door.

In the name of protecting those unfortunate women who are genuinely controlled for another’s gain, she has proposed a plan that she knows is unworkable and unfair. Its real point is that it’s designed to make men “think twice about paying for sex”. All men with all prostitutes, in effect. A virtual ban. What she wants is to deal with the “demand side” of prostitution: if only men didn’t demand sexual services, there wouldn’t need to be any nasty supply. The otherworldliness of this was bettered only by Baroness Warnock’s recommendation last week that rather than use prostitutes men should masturbate – a quaint variant on “let them eat cake”.

The justification Smith gives for making men think twice (and go home to follow Warnock’s advice) is that “the majority of women don’t want to be involved in prostitution”. How slithery. Her proposed law is supposed to protect a particular group of bullied prostitutes, not all prostitutes. Now suddenly we hear about a majority of women who don’t want to be involved with prostitution.

Which women? Which majority? And what about the freedom to choose of those women who do want to be involved in prostitution? Niki Adams of the English Collective of Prostitutes is sceptical about the home secretary’s statistics. She believes that most prostitutes do not work for pimps or traffickers. “The government figures are completely fabricated,” she claims. Even the Home Office estimates that of 80,000 prostitutes here, about 4,000 are trafficked – an unacceptable but still small minority.

If Smith’s main motive were to protect the most vulnerable prostitutes, there is a way to do it. All prostitutes should be licensed and all should work off the street and only in licensed premises run by licensed people. This would have the side effect of legalising prostitution, which many would regret.

However, it would have huge advantages: if every prostitute had to get an up-to-date licence showing her photograph, birth certificate, nationality, licensed place of work and registration with the police and show it to every punter to prove she was not under duress, many of the worst traffickers and pimps would be forced out of business. The punter could have a photocopy of his prostitute’s licence to protect him in case of any future prosecution.

Once again this government is trying to override common sense, human nature and personal freedom in the interests of a policy not fit for purpose. Judging by Smith and Harman, if there’s one thing worse than the man in Whitehall who knows best, it’s the woman in Whitehall who knows best.

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