So far it looks much like a conventional disco. A trio of blondes writhes together like seahorses in the middle of the dance floor. The crowd is mostly young and attractive, mid-20s to late 30s, and well dressed. Almost no one is talking, although we think they are giving each other, and us, hot, hard looks.

Every now and then a group pushes through the throng around the bar and disappears past the toilets. After a second round of champagne (cost $40) we step into a low-ceilinged labyrinth bathed in soft red light. The first room features a St Andrew's cross with shackles and straps for bondage and at least seven naked bodies squirming together on a raised dais. Two standard man-woman ensembles couple in hushed tones. A small young man, standing at the edge of the dais, gently and quietly works away with a brunette whose face is buried between the stocking-clad thighs of another woman. Through another doorway at least six couples in various stages of undress and congress are spread across a large, raised mattress. In an alcove, a young woman is huddled over a pale, plump male figure and a grey-haired man, at least 50 years old, reaches out to run a hand paternalistically over her buttocks as he walks past. The heaviest action is in the back two rooms, which are packed with fully clothed spectators. It's rather like the crowd around the Mona Lisa in the Louvre: reverent, hushed, jostling. Half a dozen couples are at work, but the stars are a large woman in suspenders who groans throatily and theatrically while being attended to from behind by a balding, gingery man.

Scenes such as this are common in France these days, a country that is on the verge of rendering public fornication and infidelity socially acceptable. It is not quite legal, but it is already fashionable. Echangisme, or wife-swapping to use the obsolete term, was once the preserve of bourgeois elites and middle-aged couples bored with their sex lives. But the winds of democracy and commerce have blown through and it has gone down-market, dropped its prices and got younger and better looking. Brickies and grandparents do it, but twentysomethings are moving in. "Sex is not just about a man and a woman, it can also be about two women, a man with two women, or two men with a woman," says Cedric, a 20-year-old restaurateur. "You can put on a sexual production, but with lots of sensuality." Cedric met his girlfriend, Stephanie, a nurse, at a party two years ago - a swingers' party. He arrived as part of a threesome and she was with a boyfriend. But they got to talking and he later asked her out. Things went from there. They are expecting a baby in June. "In my circle of friends everyone does echangisme," says Stephanie, 27, a slim and attractive blonde. The suggestion seems to be that swinging couples are liberated, open, a la mode, each free to live his or her own life.

France has more than 400 echangiste clubs, saunas and restaurants that encourage sex on the premises. Paris has about 35 clubs, yet Lyon, a provincial city with a fraction of the capital's population, has more than 20. The debauchery is not confined to big, immoral cities and seaside resorts. Gourin, a town in the Catholic Bible belt of Brittany, has a 500-year-old church, three or four bars, 5000 inhabitants and Le Starman echangiste club. Daniel Welzer-Lang, a University of Toulouse sociologist who has studied the phenomenon for eight years, calls it multisex - to have sex with various people in the same place or at the same time. Three years ago, he calculated the French heterosexual swinging population of the "echangiste planet" to number 300,000 to 400,000. Given its media profile and the explosion in club numbers, he thinks it has easily doubled. "There are people who say the French aristocracy practised echangisme at the end of the 19th century, but I think the upper classes have always been a bit outside social conventions," he says. Welzer-Lang says multisex broke out, as it did in most decadent Western societies, in 1970 with two parallel movements: the Paris-led echangisme of the bourgeoisie and upper classes, and the sexual communism of young people rebelling against social and political norms.

"Patrick and I have known each other since we were students. We've always been very open about our fantasies and our desires for other people," says Francoise, a 41-year-old mother of two teenagers who runs a hotel with her husband. "It is better to come here than to have a lover. It's less complicated here, it's just a game, and we always do it together." Francoise is wearing lip liner, a transparent black negligee with a rose pattern, lacy bra, G-string and high heels and sitting at the bar of Ray et Maya, a club just outside the Peripherique, the Berlin Wall of a motorway that shields postcard central Paris from its suburban banlieue. "Clubs like this are better for women than for men. No bloke has ever said no to me, but it often happens to Patrick," she laughs. "It's exciting to watch your wife making love to another man," says Patrick, 40, a man of medium build with a wispy Cary Grant moustache. "Tonight she wants the guy over there behind me. I'm happy that she is going to have that pleasure, even if I don't do anything tonight." Ray and Maya, a couple of retired cabaret acrobats and soft swingers, opened the club three years ago, saying they believed swingers were clean, cultivated, broad-minded and well behaved.

"Those who look down on echangistes are probably doing it with lovers and mistresses - there's no such thing as 100 per cent fidelity," says Maya, 42. "There seems to be less divorce in echangiste couples." "Echangistes go wild together," says Ray, 52. "I know couples who really adore each other, who arrive hand-in-hand, break out, and leave hand-in-hand a few hours later." One of Paris's oldest clubs is the 2+2, a family concern involving a couple of swinging grandparents, Roger Herrmann, 58, and his wife of 34 years, Chantal, 54, and their children, Marc, 34, and Isabelle, 31. "When I bought the club 15 years ago, swinging was exceptional," Roger Herrmann says. "Now its become normal. The clientele used to be really non-conformist, that's to say highly educated, intellectual, very cultivated, such as doctors and lawyers. These days you get all kinds." The 2+2, with another of the city's echangiste institutions, Chris et Manu, has been immortalised in French letters of the literary kind. Michel Houellebecq features sex scenes at the clubs in his bestsellers Elementary Particles and Platform.

Le 41, near the Pompidou Centre, run by an ex-porn star, Denise Lascene, also known as Diane Dubois, has been honoured in Michel Blanc's film Grosse Fatigue and a novel, Vacance Dans Le Coma by Frederic Beigbeder. Another publicity windfall for the echangiste planet was the publication of the huge-selling The Sexual Life of Catherine M., in which Catherine Millet, founder of the high-brow Art Press magazine, details her decades of group sex in swingers' haunts. Multisexers are categorised into sub-groups: melangists do it side by side with their regular partners; echangistes swap partners; triolists do it with an extra man or woman; and partouzers do it in orgies. There is the gang bang, as investigated so thoroughly by Millet, whereby a woman willingly, and for no financial reward, takes on a number of men. The record is held by one Kimberly Houston: 620 in an afternoon. Then there are the voyeurs. Without them, clubs would not survive. Clubs make money with inflated drinks prices and slugging single men double cover charges. Single women usually enter for free.

Welzer-Lang says that at one club just one in 10 clubbers had sex. Herrmann claims that half to three-quarters of his clients go in "flagrante delicto", although many come just to dine at his restaurant. "They don't even go to see the rooms upstairs. They like to be here, in rather bizarre surroundings."