How many of us have paid for sex? Have I, do I? These are personal questions. People don't much talk about them, except on the quiet. But in these circumstances they're hard to avoid.
The National Gallery's newest exhibition is all about prostitution and how we look at it. Often with art we can keep our distance from the subject. But when prostitution is the subject, and when the art is as direct as this, we have to make our positions clear.
The art in question would be striking anywhere. In the National Gallery it's something more. There's never been a contemporary installation work here before, and to kick off with a work by Ed and Nancy Kienholz, and with this particular one – well, it looks like deliberate provocation. The Kienholzes (he died in 1994) are a couple of American artists who specialise in highly melodramatic scenes and environments. Their works are constructed of mannequins and machinery, models and furniture, guns, lights, gunk, trash, animals, body parts, multi-colours.
They make you think of fairgrounds, waxworks, toy shops, sex shops, joke shops, Catholic effigies. Violent and grotesque, they evoke militarism or consumerism. They often make you shudder. The ingredients of their collages may be modern. Their moral tone is medieval. They look like allegories of all the most energetic deadly sins of our world. The piece that opens in the National Gallery on Wednesday is one of their largest and most powerful: The Hoerengracht, made between 1983 and 1988.
The Hoerengracht, the whores' canal, is not a real street in Amsterdam. It's a play on "Herengracht", gentlemen's canal, which is real, and a perfectly respectable address. But the city's red-light district is famous. (Recently it's been considerably tidied up.) A network of streets, where half-dressed women sit on offer behind plate glass windows, it's popular with tourists who come to ogle and gentlemen who come to pay. This public peep show is the basis for the artists' atmospheric installation.
It reconstructs, life-size, a couple of street corners. At the threshold you face into darkness, punctuated by red light bulbs and lit windows that glow out, showing up the edges of buildings and bits of street furniture: bollards, bikes propped in a stand. Enter the scene. There are leaves and rubbish under your feet, music playing. Take a side alley. Peer. Through each of the windows, you can see the effigies of half-naked women, sitting, posing in sexy lingerie, in a small shabby room, returning your gaze.
There are touches of straight illusion. One woman standing in a doorway has a cigarette with a glowing tip. But these figures are not realistic to the standard of Duane Hanson, let alone Ron Mueck. They don't need to be. They're enough to conjure up a troubling human presence. If they look very like shop mannequins with heavy make-up, then that's true to life, when life is to be spectacle, a low-rent fantasy.
And then the work has dramatic departures from reality. See how everything is dripping. Liquid streams of resin run down bodies and things, suggesting tears, sweat, cellar walls, a general suppuration of badness. See how every woman has her face framed by an attached glass box, with its lid open, making her all the more an object of our looking, focused, trapped. It's a dungeon of voyeurism.
And situated in the Sunley Room, it gets an extra shock effect. This is an area that anyone moving out of the gallery's main entrance exhibition hall might walk into. You can easily stumble out of the late Renaissance and into the The Hoerengracht's sordid gloom.
But why is this work in the National Gallery, which usually only hosts contemporary art through some residence scheme, on the understanding that the artists will somehow be inspired by the collection? Well, there is a connection, if you want to see it. A link between art, Holland and prostitution goes back a long time. Think of how many paintings from the Golden Age of Dutch Art are about prostitution.
In an adjoining room, they're showing pictures by the likes of Jan Steen and Pieter de Hooch. A hearty tug on a girl's skirt, everyone laughing. A gentle bedroom scene by candlelight. A gent and a lady at a table, making a polite transaction. The presence of these works, with their thoroughly benign and positive attitude to "love for sale", brings this whole exhibition to a question mark. Where do we stand?
The Hoerengracht isn't complicit with the voyeurism it evokes. It doesn't invite you to enjoy surreptitiously something low and naughty. On the contrary, it takes a very dim view of this red-light area. Its attitude is candidly puritan. If you're tempted to take any pleasure here, the work does its best to punish you. It confronts you with a spectacle of filth and imprisonment. The effigies are victims. Prostitution is horrible, a degradation, certainly of the women involved, probably of their clients too.
Turn back to the 17th century, and the business is treated quite differently. It presents the relationship between tart and punter as something rollicking or something tender. Everyone's having fun.
Times change, art's attitudes change too. In late 19th-century Symbolist pictures, the prostitute appears as a sex-mad succubus preying on innocent men. And a few years ago I saw a contemporary show that stressed the self-esteem of the sex-worker. Fun girl, demon seductress, social victim, efficient professional: all these images have something false. The facts are too varied. There are smart ladies serving Park Lane. There are smackheads loitering at the edges of estates, calling in a monotone: "Business. Business".
The 17th-century scenes are honest in one way, at least. They address their viewers as people who might use prostitutes themselves. The Hoerengracht is pure moral tourism. We look. We are disturbed and appalled. But we aren't asked to imagine that we might be among the payers. No, prostitution is something done by other people.
Myself, I have never paid for sex. But off the top of my head I can think of three male friends who have, and perhaps still do. When they told me, my reaction was frankly confused: shocked, disapproving, impressed. There seems to be something wrong with it, but I'm never quite sure what moral principle is involved, or which party is hurt, or in what kind of way. I am pretty sure that I don't have the nerve.
But many, many men do, evidently, or the industry wouldn't thrive as it does. I expect that includes quite a few men who visit or who work at the National Gallery. Soho, after all, is only a short step away. The Hoerengracht is a bold statement of a very partial truth. It focuses on prostitution at its most extravagantly visible. The vast invisible aspect of the trade – many more clients than workers, obviously – continues to walk by.
Wednesday to 21 Feb, National Gallery, London WC2. Admission free (020-7747 2885; www.nationalgallery.org.uk)