Some bleak facts about sex: Linda Grant says that far from being over, the revolution has only just begun

HOW much are you getting? Not enough, you're probably going to say. On a crowded train or bus or stalled in traffic, we look around at the mass of humanity and wonder if others have better sex lives than we do ourselves.

The middle-aged man, trudging home to his family in suburbia, scans the face of the young guy, for all the world looking if he'd just stepped out of an ad for Calvin Klein underwear, and the imagination is dazzled by visions of penthouses, a continuous supply of long-legged lovelies. The married woman, in her turn, sees the career woman and pines for a life in clothes unmarked by regurgitated breast milk, escorted on the town by some version or other of the Milk Tray man. And every night, between the party walls of houses, couples strain to hear the tell-tale sounds: are the people next door doing it more than we are?

Reports last week of a new American survey of the sexual habits of Mr and Mrs Average USA have suddenly turned on its head the notion of the Swinging Single. Married people are more likely to be having sex regularly than those who live alone. Only 1 per cent of married men had had no sex in the previous year, compared to 23 per cent of single men. Three per cent of married women had had no sex in the past 12 months, against 32 per cent of single women. Which should come as no surprise.

In her book Backlash, Susan Faludi claimed that the widely disseminated rumour that a fortysomething woman was more likely to be kidnapped by terrorists than get married was a plot by the right to discourage women from career development and late marriage. But now it turns out it's true. 'The sexual marketplace is entirely different for the single 40-year-old man than it is for the single 40- year-old woman,' the authors of Sex in America write. 'Quite conventional men, who do not have great wealth or power to offer, find it much easier to marry in their later years.' The odds against unmarried women finding a partner get longer as they age and the line on the graph between male and female sexual fulfilment draws wider and wider apart.

This bleak truth will now be turned upside down and inside out by commentators. The right will claim it as moral victory for marriage, feminists will start to examine the research methods. How true can these surveys ever be? Who doesn't lie when confronted with the most intimate details of our private lives, especially when, as in this case, the researcher is sitting opposite, tapping his or her pen against the clipboard?

Well, listen. Buried away in the back of the book, the very last chapter of the survey is another, more astonishing, statistic. When women were asked if they had ever been forced to perform a sexual act, 22 per cent said that they had. When men were asked if they ever forced a woman to do something she didn't want to, only 3 per cent admitted that they had. A year ago, Katie Roiphe, author of a well-publicised book exposing what she called the myth of date rape, claimed that if one in four of her friends had been raped, wouldn't she know about it? Where did this figure come from? Out of the addled brains of first- year students?

Yet here is the confirmation. The four authors (three of whom are men) are clearly staggered by what they have found. Might it be that the much-ridiculed set of rules established by Antioch College, which require verbal permission at every stage in seduction, have arisen from a valid problem, they ask. 'The differences that men and women bring to their sexual situations and the differences in their experiences of sex sometimes suggest that there are two separate sexual worlds, his and hers,' they write. And could it not be that it is within those sexually active marriages that forced sex is occurring? Yes. Twenty per cent of married women reported being made to do something they didn't want (white women more so than any other race). Is it not the case that the history of marriage has also included a history of 'date rape' as drunken husbands come back from the pub to exert their 'rights'?

On Wednesday I was at a literary festival talking about my contribution to The War of the Words, a new book on the 'political correctness' debate, published by Virago last week. A man stood up from the audience. He said that he had grown up before the war and there never was the problem between men and woman then that there is now. What could women do to get us back to this Arcadia, he pleaded? Another gent professed confusion. What was a chap to think when confronted with all these female fantasies about being overpowered by some Stanley Kowalski lookalike? How could we expect men not to rape women if they think that's what women want?

There is an old joke about rape fantasies: 'It's when Mick Jagger just won't take no for an answer.' (You can substitute Keanu Reeves or whoever this month's babe may be.) The US study shows that almost no women like the idea of being forced to do something sexually. Only one out of a thousand women found it appealing. The researchers conclude that the men who forced sex did not realise how coercive the women thought their behaviour was.

More than 30 years ago the development of contraceptives designed for and controlled by women ended a long period in which men set the sexual agenda and more or less everything else - women can only get into positions of power, and so influence how the country is run, if they are able to control their fertility. Since then, in the private sphere, women have been just as confused as the opposite sex as they have struggled to test out ways of getting their sexual needs met. Sex in America reveals a sexual revolution that, far from being over, has only just begun.

'Sex in America' will be published here by Little, Brown next month. Linda Grant is the author of 'Sexing the Millennium', published by HarperCollins.