Yesterday's newspapers reported a couple caught having sex behind the altar at St Paul's. What in heaven came over them? John Walsh has given careful thought to sex in public places, its geography and motivation. Is it men or women who most like going public? Our man is in no doubt.
"There is a time and a place for everything," said Mr Dave Gardener, a London plumber and freelance moralist, of the events that he had the misfortune to witness, "but we were in a place of worship". St Paul's Cathedral may well have been originally conceived as a place of worship, but over the years its profile has changed: it has become a tourist promenade, a ritzy marriage chapel and a contribution to the metropolitan skyline - and, to a certain tendency, it is one of the key sites for le bonque publique.

Mr and Mrs Gardner discovered this the hard way. On a visit to the noble church with their small children, they wandered round the back of the secluded American Memorial and discovered a twentysomething woman sitting astride a man on the floor. "It was absolutely outrageous," reported Mrs Gardner, displaying an attention to detail that marks her out as an expert witness. "Her black leather coat was open and black top raised up to her chin. Her skirt was round her ankles. She was on top of the guy whose trousers were also down ...".

Shrewdly deducing that they had not stumbled upon some unusually intense attitude of prayer as recommended by the Old Testament or the Koran, the Gardners blew the whistle on the deshabille fornicators. A woman attendant came and kicked them out; and as they left, the Woman On Top in the disarrayed monochrome actually apologised. "We're so sorry," she reportedly said.

Which leaves us wondering: why do they do it? Why do people want to have sex in public? The Kama Sutra is silent on the subject (though I haven't consulted it in, ooh, weeks) but basic brain-to-groin logic will tell you that the sexual advantages of having it off in places where the great British public may stroll by at any moment are frankly inscrutable. Were the couple in St Paul's overcome by lust? Driven mad by the combination of nave and rood screen, did they feel a simultaneous throb of desire which had to be dealt with right that minute? Or was it that they felt compelled to break the incensey, sacerdotal atmosphere of the cathedral with some primordial frottage?

Nobody can tell. But according to a National Sex Survey conducted in The Mirror, making love in public is the most common sexual fantasy among young Britons of both sexes. Doing it on the bus, in the road, on the table at the Ritz or in the midst of negotiating a mortgage extension at one's bank seems, rather bafflingly, to be a very British thing. Mostly it remains a fantasy, though there are memorable moments when reality raises an ugly head. Last February, the audience at Jolson at the Victoria Palace Theatre were treated to a virtuoso display of unscripted shagging in a private box near the stage. A shapely blonde hitched up her elegant evening gown, sat astride her portly middle-aged escort and proceeded to bounce up and down with gusto before turning to face the stage and remount his lap ... "I could scarcely believe it," said a traumatised Brian Conley, the show's star, who was singing "California Here I Come" at the time. "They were going at it hammer and tongs and didn't seem to care who saw them. When she started shouting "More! More! More!" I thought she appreciated my performance - but it was his."

St Paul's Cathedral, Poet's Corner at Westminster Abbey, the bandstand in Hyde Park (where John Getty Jnr, he of the severed-ear kidnap, was once discovered with a girlfriend in Hugh Grant mode), the Reading Room of the British Museum, the Strangers' Gallery in the House of Commons - they are all places targeted by al fresco shaggers, as surely as are the first-class lounges of aeroplanes. I know only one person who has ever joined the Mile High Club - but several who have achieved some form of penetration on train journeys from Waterloo to Wimbledon (change at Clapham Junction) and thus joined the slightly bathetic Yard High Club. But I digress. According to friends, the most satisfying place to make love in public, for its lighting, its spectacle and built-in drama, is or was the Death of Nelson exhibit at Madame Tussaud's. Backlit by flaming sails and punctuated by musket fire, one could get up to all kinds of mischief without being seen. "There used to be a cardboard cut-out of waves," remembers one shameless horizontale of my acquaintance. "You could lie down behind that and not be seen in the darkness, unless they caught a glimpse of thigh among the explosions, and then people would just imagine it was a dead sailor."

Do you remember Woody Allen's nervy little face in Everything You Always Wanted to Know About Sex ... when he plays an Italian who takes a wife and discovers she is frigid except when shopping and wants to have sex in more and more inappropriate settings (in the antique dealers, the couple disappear behind a huge mahogany dresser, and after a ferocious banging noise reappear magically reclothed 10 seconds later)?

I suspect that public sex is far more frequently instigated by women than by men. It's a girl's thing - showing off their wild streak, wanting to cause mild outrage simply with the spectacle of their being pleasured senseless. Men have to get used to impetuous girlfriends assaulting them in the street and suggesting some unfeasibly frank display of sensual gratification. A lot of men aren't so keen. They would probably admit to having a little trouble becoming aroused in church or the Savoy Grill with half the population of London seething around them. But maybe I'm just pusillanimous. Heroes like Lord Byron wouldn't give a tarradiddle for such things. He revealed his own fondness for le bonque publique in a letter to Douglas Kinnaird in 1819, praising his own epic poem Don Juan: "Could any man have written it who has not lived in the world? And tooled in a post-chaise? In a hackney coach? In a gondola? Against a wall? And under it?" Whew. And you can bet a cathedral to a condom he wouldn't have apologised when kicked out by the attendants.