Sex and marriage go together like a chocolate mousse and cabbage, if popular culture is to believed. Dissatisfied infidelity and marital shagging-fatigue are the rule rather than the exception in TV dramas and soaps. But these misrepresentations blame marriage for things that are not its fault.

The definitive British Sexual Survey (the one Thatcher tried to ban) proved that infidelity and frequency of sex are more to do with age than marital status.

While more than half of 16- to 24-year-olds had been unfaithful in the past five years, only seven per cent of 45- to 59-year-olds had.

The same trend was found for frequency of sex within marriage. The average wife in the younger age group was getting it six times a month, dropping to twice monthly in the older.

If middle age, not marriage, breeds sexual torpor, so does familiarity. A large survey of women showed that on average, the rate of sex has dropped to twice a month in relationships after the third year, regardless of marital status. Even the young went off sex if they stayed together for that long.

The problem is not these realities, but the grotesque distortion of expectations about sex that the media creates, especially among women. From before puberty onwards, their magazines are telling them that sex - and by implication, their appearance - is everything. Not to have a boyfriend with whom you do all manner of exotic erotica is to be a failure.

Girls start sex ever younger, have greater numbers of partners and fewer inhibitions. By the time the modern girl hits 30, you might imagine she would be shagged out, but her expectations of what marriage will bring remain inflated. There is difficulty in adjusting to the idea that sex might become primarily a means of reproduction and in middle age, a form of companionship.

Having been encouraged to use it as a drug to boost mood or self-esteem, the woman is encouraged to believe that such a peaceful role for sex is a sign that she is being swizzled, that she is not getting her fair share of life's bounty. Just as she may feel incredibly let down by what her career has not delivered, in terms of income and fulfilment, so marital sex does not come up to scratch.

There is a constant bombardment of media pressure to be dissatisfied with all aspects of her lot, from her house to her holidays, as well as her sex life. The idea that she might say to herself, 'I'm satisfied with what I have got - it may not be perfect but it's mine,' is inimical to the American, consumerist version of Advanced Capitalism which has colonised us. If she divorces for lack of sex, she will have no idea that her dissatisfaction has anything to do with ideology.

Of course, it's great that women's sexual needs are now far, far better met. It just seems a terrible shame that they have been hijacked as part of capitalism's need to create dissatisfaction in order to flog more products.

Next week, I'll be looking at men, divorce and sex.