No matter how soothing or how frequent the reassurances, there is something bone-chilling about the decision to extend sex and relationship education (SRE) to primary schools. In a couple of years from now, children as young as 7 will be subjected to discussion of the personal and the intimate - just the kind of discussion that has been provided for teenagers for years, without any noticeably positive contribution to their welfare.

The only way to protect a child from such blandishments will be for individual parents to withdraw him or her from the lessons. Tricky, that one: making your child “different” from the classmates. Nevertheless, if mine were still young enough to qualify, I'd grab her under one arm and head for the hills.

It's not the teaching of the nuts and bolts that is alarming. There is no sensible reason why a child of any age may not know which bit goes where; indeed, stick it to them as ruthlessly and factually as is possible, and there's a fair chance that many will deduce - correctly - that the whole business sounds messy, ridiculous and best avoided for as long as possible.

Besides, tens of millions of children from farming communities have witnessed, since birth, the what-fits-where and have neither aspired to emulate bulls by the age of 8, nor grown into teenage studs and hussies; in fact, in many such rural areas - take the buttoned-up American Bible Belt, for example - the reverse is the case.

No, induction to the biology of sex holds no horrors; schools are there to teach ologies. But the teaching of “relationships”? Expanding the national curriculum because, as Jim Knight, the Schools Minister, put it on Thursday “We need to improve the framework and moral understanding within which these things are taught”? It is effrontery at best, unworkable at worst, and places unspecialised teachers in a position that neither we nor they deserve them to be.

The whole point of the national curriculum is standardisation, the better to pursue the principle that every child, in every school, is exposed to the same finite slab of objective knowledge and instruction therein.

Yet it is hard to think of anything more subjective than any one person's idea of what constitutes a relationship, let alone a good or a bad one. For some, it is that to which you pledge yourself for all eternity; by contrast, a teenage girl explained to a newspaper this week that she only has sex with steady boyfriends - then totted up four this year. So far.

Sometimes uncertainty is scorchingly apparent, as at a rounders match I once attended, organised by the ebullient comedian Arthur Smith. “We need two teams!” he instructed the mingled crowd. “To my left: all those in a steady relationship. To my right: all those who aren't.” One woman moved to his left as, simultaneously, the man with whom she had arrived headed off to the right. Ouch.

Their problem, not ours; nonetheless, if it is possible for two sentient, educated adults not even to know whether they are in a relationship, it is difficult to see how Mr Knight's “relationship education” for children is going to be homogenised, nationwide, regardless of community or regional quirk - and more difficult still to see how thousands of teachers may then be able, even if they so wished, to teach the same lessons as each other.

How does a teacher in an upright shire impart a “moral understanding” of the polyamorous? How does another teacher, facing 30 little brown faces in Bradford, tackle a “moral understanding” of the polygamy that some of their relatives overseas still enjoy? How does a gay teacher in Brighton (I know, I know, but clichés have their place) convincingly incorporate the same “moral understanding” as a nun teaching in a Roman Catholic school along the road in Hove?

Indeed, those who applauded the repealing of the infamous Section 28, which sought to prosecute any who “promoted” homosexuality in schools, might well have cause now to fear its resurrection. For how can any government, mindful of voter popularity, regulate a teacher's handling of the potent curiosity of a seven-year-old without once more drawing the uneasy fine line between the promotion of a lifestyle and the simple answering of a question?

By the same token, any sickly child off school on Thursday morning might have idled his way through Woman's Hour, on which a gentleman of tremendous self-assurance spoke eloquently of the benefits of “swinging” between couples; in fact, he claimed, some two million Britons are at it as I write. Such a child, back at school yesterday and burning with questions, might easily have put his teacher in a predicament that a national syllabus for SRE would render impossible to handle.

At least, at the moment, without such a syllabus, the teacher has a range of choices: depending on the child, on the school, on the family and on his or her own sensitivity, he can do his best to explain - or he can take refuge by saying that he doesn't know, why not ask your dad, it's not my job, get on with your sums.

In the end, once Jim Knight and his advisers have come up with their one-size-fits-all “moral understanding”, my bet is that, rather than offering anything radical or new, it will be a watery version of the way most of us were taught about sex at home: “When Daddy loves Mummy very much...”, placing sexual contact firmly within the context of committed, caring, ongoing relationships. Same old, same old.

Except that now, instead of asking schools just to provide empirical back-up by teaching the mechanics and the science, we are asking them to involve very young children to engage with relationship conundrums that are infinitely more complex.

One may, for instance, successfully plant the idea of love and sex as mutually adhesive concepts. But it is a challenge too far then to ask children to understand the difference between “you have sex because he loves you” and “you have sex means he loves you”. Good grief, there's no shortage of grown-ups who have yet to master that; what's a teacher of nine-year-olds, quite possibly muddled in her own personal life, really supposed to say?

“Hands up if you know the answer”?

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