In the TV trailer for the Channel 4 drama Secret Life, actor Matthew MacFadyen says: "My dad did stuff to me when I was eight - I thought he must be right, I must have wanted it and then I grew up and I thought every kid wanted it."

It's a dreadful line, a shocking line. It might have a place in the full context of the 90-minute drama on Thursday, but used as a trailer it is absolutely wrong. The trailer's purpose is to promote the drama. Unfortunately, it's also selling a dangerous idea that could give comfort to any paedophile watching who can think: "Me, too; I thought every kid wanted it - and I still do."

The fact is no-one, in the teeth of all the evidence that exists, could possibly think eight-year-olds or seven-year-olds or 10-year-olds like to be sexually abused. Ask the vast majority of survivors. Ask the people who have limped through the years, plagued by depression, since their trust was destroyed and their bodies were violated.

So why this play? Rowan Joffe, its writer, says that we need a grown-up discussion about paedophilia. We need to extract it from the realm of hysteria and the language of baddies and monsters. We certainly do. But must we kick start this with a portrayal of the plight of a sex offender against children who wants to reform? Of the victims of abuse, his pain is the least compelling. As a viewer, I care so much more about the victims, about their parents, their siblings and one day their partners and children. They, too, will suffer.

On April 1 there were 3403 names on the Scottish sex offenders' register. Of those, 1270 live in Strathclyde, 617 in Lothian, 320 in Grampian and a similar number in Tayside. Chilling, isn't it? It gets worse. There are 110,000 convicted child sex offenders in England and Wales who are not on the register because their convictions precede its existence. (The figure for Scotland is not available.) Then there are those who haven't been convicted. How many of them are there? It's anyone's guess, but it is accepted that for every 10 offences against children, only one is reported. So the problem is vast and largely invisible.

Why? Why, when the wellbeing of our children is so close to our hearts, do we know so little about paedophilia? What causes it? Why is it so widespread? Can it be cured? Crucially, how do we spot it?

Now that it is growing more difficult for convicted paedophiles to find employment that gives them direct access to children, we are told they are turning their attention to single parents. In England and Wales (though not in Scotland), there is a proposal to allow women with children to check the name of a new partner against the sex offenders' register, if his behaviour arouses suspicion. Given that most offenders are not on the register, there is an obvious danger of this giving a false sense of security. So I asked Ray Wyre, who has worked with paedophiles for decades, what a single mother should beware of in a new partner.

He said that while women might have grown weary and wary of predatory males, a paedophile is likely to be viewed as the answer to a prayer. He will be charming, sympathetic, gentle, treat her like a princess and be delighted by her children. He'll come across as a family man. In other words, she'll think she's struck it lucky at last and drop her guard.

It is a dreadful irony that in the domestic arena, as in the wider world, the paedophile adopts the characteristics of the best of men. Generations of youngsters have thrived because of, indeed been rescued by, good guys who have given up their time to form football clubs or swimming clubs. Now many decent men won't get involved in youth work in case their motives are misjudged. Churches have been infiltrated by child molesters, as has the teaching profession and social work. But how have these warped men walked for so long invisible in our midst? Either they are chameleons or we display the blindness of those who will not see.

There are tell-tale signs. Wyre says that, unlike a real family man, the paedophile may soon show more interest in the children than in their mother. He may be gender specific; preferring to play with the boy than the girl or vice versa. He may, rather quickly, start to take over aspects of the child's life that should be their mother's responsibility - and that most men would feel uncomfortable with. He might start to buy the children clothes, especially underwear, or to get involved in sex education. His aim will be to get the child alone, so he will try to find ways of encouraging the mother out of the house and to isolate the children from one another.

Most children respond to generosity and attention. It makes them feel special, and the paedophile's method is seduction. He uses treats and attention to lure the child into a secret world where sex is introduced as a grown up activity that the mother is keeping from the child. Children invariably have secrets from their parents: be it a sneaky cigarette or buying forbidden sweets. This becomes one more secret, the biggest one. Besides, they have no vocabulary to speak about it.

And it gets worse. For, while girls do not go on to become abusers in adulthood, according to Wyre, it is boys who have been seduced into being abused who do. And so the cycle turns once more.

So, what can be done? These men don't look or sound like monsters, but monstrous their behaviour remains. Wyre points out that offenders are still getting away with abusing children for 20 or 30 years. How do we get a grip on the situation without resorting to hysteria and vigilantism?

The register is a start. But if it is necessary to check all would-be employees' names against it, why not also the self-employed who work with children and why not men entering relationships where they will have access to children? Why not make it an offence for a man who has been convicted of a sexual offence against a child to do these things?

Men who abscond from their registered address or refuse to co-operate with the authorities can now have their name and picture posted on the net. If their behaviour gives concern, police can tip off a school, leisure centre manager or a householder. These are all moves in the right direction, but more is needed.

Secret Life was inspired by the closure in 2002 of Wolvercote, the last residential paedophile rehabilitation centre in Britain. In Scotland, sex offenders receive treatment in Peterhead prison and there are treatment programmes in the community. But there is no residential facility. Yet Wolvercote cut reconviction rates by between 40% and 50%. Donald Findlater, who ran Wolvercote, insists that while some paedophiles are so dangerous they should remain locked up, many more hate what they have become and the hurt they cause. They want help.

Our fear, distaste and desire to protect our children mean that no-one wants a rehab centre in their neighbourhood. They also mean that we avert our gaze from the entire subject when what we need to do is expose it to scrutiny. There are large numbers of child sex offenders in our midst, whether we like it or not. Hysteria is neither proof against the situation nor a solution. We need to exercise intelligence and compassion; to deal with the issue.

We need treatment centres. We need research. We need to familiarise ourselves with the characteristics of an abused child. We also need to know what danger signs to look for in a charming adult. As Wyre puts it, we need to open our eyes and think offender.