Hot on the heels of the debate about the liberalisation of cannabis sparked by Ann Widdecombe comes another debate, which also pitches liberals against conservatives, this time about sex on screen.

Earlier this year, the film censors - the British Board of Film Classification - published new guidelines which allowed for more explicit representations of sex on screen in 18-rated films than ever before, on the grounds that adults should make up their own minds about what to watch.

But not everyone is happy with the new, more liberal climate. Among those with deep reservations is the Home Secretary, Jack Straw.

The differences between the two sides have come to a head over a particular category of videos: those given an R18 certificate, which allows them to be sold only in licensed sex shops.

Three years ago, the classification board relaxed the rules on the content of R18s, on the grounds partly that you could buy more explicit sex education videos, classified 18, in high street stores.

It decided to allow scenes which are still very tame by most European standards.

The Home Secretary objected when the police and Customs and Excise pointed out that some of the material the classification board was now permitting would be seized as obscene if someone tried to import it from abroad.

The board was forced to backtrack. Some of the R18 certificates were withdrawn. The distributors of one of the videos, Makin' Whoopee!, appealed to a body called the Video Appeals Committee.

The committee decided the video was not obscene and should be granted an R18 certificate.

Seven more explicit sex videos were also refused certificates. Their distributors also appealed, and the committee again found in their favour. It took the view that there was little risk that children would see the videos, and limited evidence that they would be damaged if they did.

The classification board sought judicial review of that decision. The judge decided the Video Appeal Committee had acted properly, and the classification board has now resumed licensing explicit R18 videos - although there are still limits on what they can contain.

The classification board won't licence anything which breaks the law, which might encourage paedophilia, which involves inflicting real or simulated pain, or which is "degrading or dehumanising".

Jack Straw still isn't happy, and he's now issued a consultation paper on tightening the controls once more.

The government, the consultation paper says, "takes the common sense view that exposure to pornography of this type is potentially harmful to children". It is especially worried that explicit sexual material of the kind now being permitted at R18 can be used by paedophiles to "groom" their potential victims.

The Home Office suggests a number of ways of tightening the rules. One would be to change the definition of a video's "potential viewer" so that the classification board, when issuing certificates, must take account not just of who is "likely" to view the video but who "may" view it.

It also suggests creating new offences of showing an R18 video to a child and "failing to take reasonable care to prevent a child from watching an R18 video".

Mr Straw has received responses from two video industry bodies and from the classification board - and he is likely to find the responses disappointing.

The classification board, while sharing concern about the exposure of children to pornography, concludes that several of Mr Straw's specific proposals either would not work, or are not necessary.

The new offences - given that they would take place in the privacy of the home - might be unenforceable. And it would still not be an offence to show unclassified pornography to a child.

And it also says there is little expert evidence that exposure to pornographic videos would do most children harm.

The British Video Association, representing mainstream video distributors (NOT the distributors of R18 titles), and the Video Standards Council (which draws up the industry's own code of practice) are more forthright in their criticisms.

The British Video Association says the real problem lies not with the small number of R18 videos (29 out of more than 5,500 titles certificated), but with illegal pornography.

The Video Standards Council turned to the eminent barrister Geoffrey Robertson QC for an opinion. In his view, Mr Straw's suggestions for a new offence of failing to take reasonable care to stop children watching R18 videos would authorise police snooping on private houses; he calls that an Orwellian prospect. .

The video industry thinks that in concentrating on R18 videos Mr Straw is ignoring the far greater problem of unlicensed, uncertificated and illegal material.

Jack Straw doesn't sound as if he's convinced by that sort of argument in the context of soft drugs: he's unlikely to be any more convinced when it comes to sex on screen.