Claudia Pritchard: How do you police this, Ms Lumley?

When Joanna Lumley rounded on the morality of the modern British child last week, she was on safe ground in condemning shoplifting, cheating, and playing truant. But she steered well clear of an area in which behaviour is less obviously right or wrong. When, in short, if ever, do you start letting boyfriends and girlfriends spend the night in your child's bed?

Imperceptibly, over the years, the place of safety that used to accommodate only Mr Snuggles, Cattypuss and your nipper, holds more arousing possibilities than a rereading of The Hungry Caterpillar. The time comes when the pillow talk is not with Mummy or Daddy, and the subject is X-rated. And this is the cue for a parenting skill that is not in the handbooks. The choices are many, the guidance non-existent. You are on your own, and it is not as simple as teaching your daughter or son not to steal.

Many of today's parents of teenagers, as young people, would not have been allowed to share a room until they were married. I know of a woman in her thirties, who has been married, who still cannot stay with her boyfriend in his parents' house.

But it is increasingly common for boyfriends and girlfriends to stay, and from an ever younger age – and they do not welcome a room of their own. I know of 14-year-olds who are allowed to sleep together in the homes of parents who are described as "liberal". But even other liberals might baulk at an arrangement that risks prosecution for facilitating under-age sex. At the other end of the spectrum, some parents, possibly over-anxious, will not let six- or seven-year-old boys and girls muck in together at a sleepover.

Some take the line that, if the young are choosing to have sex together, better this be associated with a place of comfort, affection and, indeed, other refreshing possibilities, such as music, conversation or real food. If the alternative is a functional bunk-up in a lay-by, that is probably not creating the foundations for mutually fulfilling, considerate sensuality.

But there is really no such thing as a young, dependent couple: by the time you take into account a parent or two, some younger siblings, perhaps, and an elderly relative also staying or living in, it is getting quite crowded in this relationship. Turning a blind eye to the youngsters who close the bedroom door behind them is one thing; explaining to your 11-year-old that he can't bounce in on his big brother is another. As one father says: "It would be easier if you could send them off to the east wing."

In deciding if and when it is appropriate to let the young stay together, they rely on a moral certainty that Joanna Lumley might have named: all concerned have to tell the truth. The parents who check that a girlfriend is allowed to stay run the risk of offending someone, or everyone. If they ask but do not check with her parents, they may be withholding information they should share. If they ask and do check with her parents, a) the girlfriend may be offended that her word is not trusted; b) so may her parents; c) they may be baffled to be consulted at all, or d) appalled that the stay was even contemplated.

And if it is considered reasonable for long-term girlfriends and boyfriends to sleep together at home, what is the definition of long-term? Seven years? Six months? Two days?

While many of us have discreetly accepted our children's sexuality by tacitly allowing stopovers from the late teenage years, few are prepared to see a new face daily at breakfast.

In an age when it is not financially feasible for the young to move out at the earliest opportunity, parents have a new moral highwire to walk. Lean too far one way, and you send out curiously negative messages about love and passion; too far the other, and you unbolt the family's moral scaffolding. Everyone has to devise their own route – and even the divine Ms Lumley cannot help.