What is it about sex crime that turns liberal politicians into legal vigilantes?

The Justice Secretary, Kenny MacAskill, is as liberal as they come. He wants to abolish custodial sentences for many crimes; he released on compassionate grounds the man convicted of the Lockerbie bombing, defying even President Obama; he supports the European Convention on Human Rights and the presumption of innocence. Kenny MacAskill is also a passionate defender of Scots law against the predations of the "English" Supreme Court. Except when it comes to sexual offences.

Too many rapists are going free, he believes, because of the archaic and patriarchal law of corroboration - the legal principle that, to secure a conviction, evidence must come from more than one witness to a crime. It's a principle that has underpinned Scots law for a thousand years and is defended by almost every legal authority in Scotland.

The former Solicitor General, Lord McLuskey, says abolition of corroboration will lead to miscarriages of justice. High Court judges, the Law Society of Scotland and the Faculty of Advocates agree. The Scottish Human Rights Commission made its view clear in a statement in March: "Corroboration plays an important role in Scots law in preventing an accused being convicted on the basis of evidence of insufficient quality". Of 528 Scottish lawyers polled by Scottish Legal News, only 3% thought the move would "strengthen and improve" Scottish justice.

Of course, lawyers can be wrong. They frequently are. But it's rare for so many to be so wrong all at once. Yet, it is assumed to be self-evident, by politicians and women's groups, that there is something peculiar about the crime of rape that requires a different kind of justice from the one lawyers believe in. But is rape really worse than murder? Is killing 287 innocent people in a jet airliner somehow less serious than forcing a woman or a man to have sex?

Now, I am on dangerous ground. If you question the abolition of corroboration you are likely to be accused of defending rapists. Police and victims groups are adamant that too few rape cases are brought to court because of the difficulty of finding witnesses to an offence that, almost by definition, takes place in private. Instead, they want defendants to be found guilty on the basis of the victim's evidence alone. In other words, if a woman says she has been raped, she has been. End of story.

I find this disturbing, and not just because I'm a man. Women are guilty of sex crimes too. There is the not insignificant problem of homosexual rape where the defendants can be male or female, and even female rape of men is not unknown. Anyway, justice should surely be blind to gender. The reality is that we live in a sexually promiscuous culture where, every weekend, hundreds of thousands of sex acts occur - not always under the influence of alcohol - in which consent is assumed but not explicit.

Consent is a perishable contract and can easily be withdrawn retrospectively in the cold light of day. People in the flush of sexual attraction rarely feel the need to consult lawyers before going to bed, or to secure statements to confirm that consent has been given. I do accept that too few rape cases are brought to trial, though I don't see how relaxing standards of evidence can solve this problem. I accept that corroboration is not universal in Europe, and countries such as Holland have other means of avoiding miscarriages of justice. However, as the Law Society of Scotland points out, corroboration is the basis of Scots law with good reason: that, for any conviction to be safe, it should be based on more than the testimony of the victim alone.

It's hardly surprising that the police want to abolish corroboration because it would greatly enhance their chances of conviction, not just in rape cases but in countless other crimes. The police have a vested interest in lowering the bar because it means they won't have to bother so much looking for evidence. But we all know where that leads: back to the days when police, seeking to improve their clear-up rate, fitted up people for crimes because they "looked guilty". Or because the accused has made a "voluntary confession".

The Scottish Government has a vested interest, too. The SNP are keen to attract votes from women, who have tended to be unsympathetic to the Nationalist cause. Hence the focus on allegedly "women's' issues" like child care and domestic violence. The Scottish Government is pushing this measure through Parliament on the basis of its inbuilt majority in a parliament with no revising chamber to reflect on the quality of the legislation.

But the strongest argument against abolition is that there's no guarantee it will actually lead to more rape convictions. If juries are increasingly presented with "he said/she said" cases they're likely to start making their decision on the moral credibility of the parties involved. An unemployed single parent on Benefits Street accuses a married man of raping her. Are juries going to convict on the basis of her claim alone? A woman in a divorce case retrospectively accuses her husband of rape. Are juries likely to side with her just because she claims to have been coerced? It is no longer necessary in rape trials to demonstrate physical violence.

Like the unworkable Offensive Behaviour at Football Grounds and Threatening Communications Act. the Scottish Government is using the law to make a political point. We are in a moral panic about historic sex crimes following the Savile revelations, and politicians want to be seen to be doing something. We may not like soap stars and ageing disc jockeys, but even they have a right to a fair trial.

Yet it is standard practice for the press to report sex crime allegations as if they were fact. Take a tabloid headline: "Corrie Ken Abused Girl, 14, in TV Loo". If corroboration is abandoned, and the number of trials for rape increase, and with it sensational press coverage, the case for anonymity for the accused will become unanswerable. I'm not sure that is what the abolitionists want.

Some victims groups seem to believe that this kind of press coverage is a good thing because it punishes the perpetrator irrespective of the trial outcome, and also encourages other victims to come forward. But that isn't justice; it's a witch hunt. It is deeply worrying that sensible people can be so casual about discarding the rights of the accused and our protections against wrongful arrest and conviction.

I am also worried about the silence of those many commentators, female and male, of liberal conscience who seem reluctant to talk about this issue, presumably afraid they will appear to be condoning violence against women. Well, I hope no-one is going to accuse me of anything like that.

But if so, I'll just have to live with it, because I'm afraid the Scottish Government has lost the argument. Scots law has been around a lot longer than Kenny MacAskill. The best thing would be for the Justice Secretary to admit that this is a populist bridge too far and get back to reforming prisons.