Joan Smith: Now we might discover the truth about paid-for sex

A man who uses a prostitute must be sure she is acting of her own free will

A drinking buddy tells you about this flat he's been to where you can have sex with fantastic girls.

They're in their teens and they don't speak much English, but they'll do whatever you want for £20. He gives you the address and you set off, not giving much thought to why the girls have left some country you've barely heard of: Moldova, where's that? You pay your money, the girl doesn't say much but you're not there for conversation. What's wrong with that?

From today, when a new law comes into force, the transaction could cost you a fine. If the "girl" you've just had sex with is being controlled by pimps or sex-traffickers – subject to "force, threats (whether or not relating to violence) or any other form of coercion" by a third party, in the words of the new Policing and Crime Act – you have committed a criminal offence. And you can't complain you didn't know because the law imposes a duty on men who buy sex to establish that women are acting of their own free will.

Until very recently, men who pay for sex have been treated as though they have no moral responsibility for their actions, whereas women who work as prostitutes have been subject to harsh condemnation.

In his new memoir, the writer Christopher Hitchens describes a visit to a brothel in New York with Martin Amis – for research purposes, he explains – when he was saved from going through with the act because the "avaricious bitch" asked for too much money. That was many years ago, and there's no suggestion that the woman was trafficked.

Characterising women who sell sex as mercenary is one way for buyers to avoid feeling guilty about what they're doing. Another is pretending that most prostitutes are like Billie Piper in the TV series Belle de Jour or her real-life inventor Brooke Magnanti, the researcher who wrote a blog and several books based on her experiences as a call girl. These cultural stereotypes are very important because they keep the focus on supply, supporting the myth that prostitution is in essence a question about women and barely about men at all.

This is nonsense. Last month two sex-traffickers, Andrea Novak and Joszef Budai, were jailed for eight years after one of their victims kept a diary about her experiences while working as a prostitute in London. The 19-year-old Hungarian woman described having sex with so many men as "unbelievably painful" and couldn't bear to write about being forced to pose for photographs which were to appear on a pornographic website.

One day a "horrible man" came to the flat in Belsize Park, north London, where she was locked in with another woman: "I told him I did not want to do the sort of sex he wanted but he did not want to listen... My tears were falling as I was lying there."

This is the responsibility that men who pay for sex do not want to acknowledge: they have created the demand that makes sex-trafficking a multi-million-pound worldwide business. There would be no point in transporting girls and women across countries and continents if there were not a pool of men willing to pay for sex with them; in the same way, there would have been no easily available victims for Steve Wright to murder in Ipswich if other "punters" hadn't drawn vulnerable young women onto an industrial estate where they could exchange sex for money to buy drugs.

From today, paid-for sex comes with a new obligation which requires buyers to make sure they're not enriching pimps, traffickers or drug-dealers. As well as challenging the notion that there's a "fair-trade" version of prostitution, it finally places the buyer and his motivation at the heart of the debate; most "punters" are married or cohabiting; they pay for sex because they want variety, unsafe sex or a transaction in which the woman is completely subordinate.

When the police start using the new law – and how enthusiastically they enforce it is another question – we will begin to see the true face of paid-for sex in this country. Don't expect it to be remotely glamorous or romantic.