There is a game that philosophy students play when considering whether all knowledge resides in the physical brain or whether there is something beyond. The game consists of supposing that Mary is a brilliant scientist, raised from birth in a black and white room, studying the world through a monochrome television. She is a specialist in the neurophysiology of vision and colour, retinal response to wavelengths, all that stuff. One day she is released from the room and sees a tomato. Is she learning anything new, or not? If not, physicalism rules. If she is, does it prove that “mind” exists independently of “brain”?

Let us leave aside the fact that you can annoy philosophers no end by pointing out that Mary’s body will - through adolescence - have introduced her to such concepts as beige, pink and pustule red. You don’t want to get on the wrong side of a philosopher with a heavy book to throw. But discussing this with my husband, the philosophy student, brought me straight to the TES survey of teachers’ views on sex education.

We have tens of thousands of little Marys (and Martins) growing up in their sexual black and white room, instructed in theory about human passion. What happens when they step out into the multicoloured world of erotic love? Some of them become prematurely familiar with the mechanics but forget the emotional cost. Others are swept up in the emotion but know so little about the mechanics that they court disaster.

The question is, how to teach them to deal with reality - the bright red tomato - when they do encounter it? How much, how early, how technical? And beyond biology, is it really a school subject?

Everyone has their memories. Mine involve being shown a diagram of a rabbit’s innards by an embarrassed nun who did a riff about what happens in the Catholic home at nightfall. “The wife goes upstairs and then the Catholic husband locks up the house (and presumably puts out the Catholic cat) and comes to her, and the act of married love takes place.” The rest I got from grimy paperbacks by Harold Robbins in the British Council’s free library in Hamburg.

The new generation does better, I suppose; personal, social and health education flourishes.

One rather bitter comment, however, came from a 22-year-old: “For God’s sake, why can’t schools teach you how to fill in a tax form? When I think of the hours and hours of rubbish about sex `n’ relationships with some geography teacher with a duff marriage . ”.

Another says: “Look, a sex description from a teacher is so startling that you never forget it, however hard you try, so why do it so often?” This school of thought says that all you need is a particularly fierce district nurse to turn up twice a year for an unforgettable hour of suppurating diagrams and warnings about child support, and leave the touchy-feely emotional bit to the English department. But even with constant repetition, other children appear to reach the age of 16 without the foggiest idea where babies (or chlamydia) come from.

But crunching through the TES survey, one figure leaps out: the statistic about training. Get this: three-quarters of those asked to teach sex education were offered no training - not even a discussion group. Frankly, if I had growing children and I knew that the poor sap detailed to teach sexuality was going to be stumbling through an instruction book or making it up, well, I might well baulk and revert to a home-based combination of books, supper table chat and soap opera civics.

This latter technique is very satisfactory: curling up with Coronation Street (or, later, Sex and the City) is very handy on the personal and social side. “Was Jason cruel to sleep with Becky when he knew it was just a one night stand but she thought she was in love? Is it making Sean happy, cruising down Canal Street, or is he really looking for someone who will love him if he’s ill or sad? Is Carrie a complete dingbat? Is Samantha wrong to use men as human vibrators? Why do you think Jack and Vera Duckworth stayed together for life?” OK, not all parents will go in for this, but more do than you would think.

The trouble with sex education, it seems to me, is not the sex. It’s the emotional baggage. It isn’t just about avoiding teenage pregnancy, abortion and disease: you can teach those monochrome basics until you are blue in the face and still the bright red tomato moment will derail young people if they have no emotional intelligence. Real sex education is about passing on the essential message (missing in a lot of adult lives, even some teachers’) that other human beings are not playthings, not masturbatory aids, not disposable. It is about teaching - and demonstrating daily - the qualities of respect, kindness and consideration. We are not shy to hammer children with moral lessons about the environment or racism, but tiptoe feebly around more everyday evils in the name of “non-judgementalism”.

Real sex education would state quite uncompromisingly that the stuff in lad magazines about “fit birds” and “scoring” is contemptible and stupid, and so is any girl who thinks of her sexuality as a commodity to be exchanged for money and flash nights out. Tough sex education need not be homophobic, or religiously constrained, or obsessed with marriage, or upsetting to children of single mothers, or in any way unforgiving of the crazinesses of love. But it has to be human: full colour and gloriously human, not the dreary, mechanistic black and white it often becomes. Perhaps English departments had better be brought in after all: “John Anderson, my jo . ”, “the marriage of true minds . ”, “Lay your sleeping head, my love . ”.

Then in comes the district nurse, snapping on the rubber gloves. That’ll sort them.

- The Big 5: Be Healthy, a four-page centre pull-out.

Libby Purves, Author and broadcaster who presents `The Learning Curve’ on BBC Radio 4.