For the past two centuries, what Mr Maclean calls "the best erotica" - that is, artistically depicted sexual intercourse - has been locked away in private collections, or museums where only educated libidos dare to seek access. The randy philistine in the street has had to be satisfied with glossy magazines containing photographs of solitary women posing as if for a proletarian Gray's Anatomy.
Mr Maclean's mission is to winkle artworks of amorous frolics out of archives and send them forth in multiples. Not too copiously, of course. Mrs Grundy or over-zealous policemen might consider an irruption of rampant, arty penises to be in danger of simultaneously breaching the British taboos of sex and class.
Nevertheless, he and his business partner, Tim Hobart, are the world's most prolific publishers of erotic art - seven portfolios in two years. Without their gallery exhibitions and publications, classic erotic illustrations by Fedor Rojankowski, Franz Marquis von Bayros, even Beardsley and Rowlandson, would have remained under lock and key.
Why erotic art? "Because it's neglected," he says. "Most artists throughout the ages - Greek vase painters, Watteau, Daumier, Degas, Picasso - have produced erotic art, but much of what survives is still unknown. Discovering it is like opening the last drawer in a chest. You think: 'That's fascinating. Now that I know the full range of his art I see it differently.'
"I like erotic art because I think there's a link between the enjoyment of art and the enjoyment of sex."
Eton? "Nothing to do with it. It was mundane and unedifying - from the sexual point of view as well."
With his business suit, balding head and privileged schooling, you might spot Mr Maclean straight away as a devotee of erotica. It is a little more surprising to discover that he is equally passionate about the early 19th-century landscape oil sketches in which he deals - dashed off in the open air by artists such as Pierre Henri de Valenciennes, before the Impressionists made an industry out of well-pondered slapdash. Such lightning sketches have their own seductive charm - they were created in moments of abandon and usually remained in the artists' hands. Hardly any were sold until Constable's day.
It was just such an abandoned moment that set Mr Maclean upon the trail of erotic art. He had suggested to Mr Hobart that they mount an exhibition of neo-classical design in their gallery in St George Street, London. Mr Hobart laughed disdainfully. Piqued, Mr Maclean retorted: "I suppose you'd prefer an exhibition of erotica."
The result was the ground-breaking exhibition, Forbidden Images, in 1985. It was a sell-out. Mr Maclean had taken the precaution of seeking legal advice, fearing prosecution for obscenity. His lawyer suggested simply that the gallery's window blinds be drawn - an expedient that had less to do with obscenity than the risk of causing overcrowding on the pavement in contravention of the Carriage Acts. He has since held three more exhibitions in London, one in Bologna and one in Sydney.
The year after Forbidden Images, Bonhams held its first sale of erotica. The auctioneers removed some lots upon advice from Scotland Yard. The Yard said that conjoined genitals were okay, legally, but children, animals and cruelty were out. Bonhams' annual auctions ended after five years because it could not find enough erotica to fill them.
Mr Maclean - whose erotic art has never provoked any brushes with the police - solved the problem of short supply by reproducing. His first edition was "the only explicitly erotic pop-up book ever made", David Russell's contemporary The Secret Carnival, "a wicked expose of decadent fun at the Venice Carnival", priced pounds 250.
"It was a nightmare. There were 13 pop-up penises in each of the 320 copies - 4,160 in all - that had to be glued in by hand. We had to hire teams of art students to do it. Most were women. No, they did not mind. They were paid quite well." About 100 copies remain.
Then came 500 copies of Paris Spring 1933, a set of 30 art-deco coloured lithographs, the chef-d'oeuvre of the Polish artist Fedor Rojankowski, nicknamed "Rojan", best known for his children's book illustrations. The original 1933 edition sells for pounds 2,500 a copy at auction. The Society's cost pounds 225 each.
Paris Spring shows an encounter between a man and a woman in the Metro. They are "soon passionately entwined in a taxi ... which delivers them to the maison de passe where they may consummate their mutual lust in more privacy and with less frantic urgency". The Society has published editions of Beardsley's classic Lysistrata and Suzanne Ballivet's Initiation Amoureuse, soft, romantic pictures of a Bardot-like nymph on her honeymoon, probably published in 1958 but dated 1943 to confuse the authorities. The 2,500 copies are priced pounds 95 each. Coming soon: Achille Deveria and Henri Grevedon's illustrations for de Musset's Gamiani ou Deux Nuits d'Exces.
The latest edition of Mr Maclean's quarterly Erotic Print Society Review, which has 6,000 subscribers, includes an investigation of Watteau's erotic art by Peter Webb, author of the erotica collector's bible, The Erotic Arts, a confession by the London antique dealer, Alistair Sampson, describing how a madam upbraided him for "interfering with the livelihood of my girls", after catching him collecting prostitutes' cards from a telephone box, and a brief account of abduction by aliens.
With some delicacy, Scotland Yard told me that within its area, central London, the Society's portfolios "would be capable of being obscene" under the law. But there would be police action only following a complaint, and then it would be up to the courts to judge. The spokeswoman added that section 4 of the Defence of Public Goods Act could be cited to defend publication of works of science, literature, art or learning. To Mr Maclean's dismay, the unshockable art market considers erotica frivolous. He has had to make the Erotic Print Society a separate company from Hobart and Maclean, which sells, among other paintings, landscape sketches: "Otherwise, buyers might not take us seriously, especially museums with their gravitas."
Mr Maclean's wife, Sarah, entered the room as I was giving her husband a hard time on behalf of Mrs Grundy. He and I had speedily resolved, in commendably adult fashion, the distinction between erotica and pornography: one degrades and exploits sex, the other is a celebration of sex by an artist.
"Only people outside the art market are shocked by erotic art," Mr Maclean said, "and only one woman, at a dinner party I attended, has ever told me 'I find that offensive' when I told her what I did. Even feminists no longer treat it with unthinking denial.
"Why some people are alarmed by it, I cannot say. I do my best to tell them it's marvellous or interesting or amusing. Most people are fascinated and want to know more."
Mrs Maclean chipped in: "People who find erotic art disturbing worry me. The last thing they want is for their nanny or cleaner to find it lying around."
Mr Maclean shifted in his seat and made a hurrumphing sound. We were penetrating another taboo: sex and class. "That does sound a bit like the judge in the Lady Chatterley trial," he told his wife, disapprovingly. "You're talking about working-class puritanism. Not a subject I like to get involved in. Sounds so patronising."
However, Mrs Maclean was warming to her subject. "I once showed our catalogue to a building worker here, a real sexist, very much one of the wet T-shirt brigade. He was horrified. Just couldn't handle it."
As I left, I noticed a parchment lampshade decorated with fluent Indian- ink drawings of cunnilingus, woman-on-top and a woman appearing to inspect her genitals with the aid of a mirror.
"Who drew those?" I asked.
"I did," said Mrs Maclean.
On the way home, I came across the last illustration in the Society's catalogue: the proposed publication of a contemporary crayon drawing by Georges Sainte-Croix of woman-on-top penetration in a household bath, viewed from the rear: a gentle, whimsical portrayal of what could have been crudely anatomical. The tub and taps were primitive. Could the couple possibly be working-class? I found myself hoping so.
Erotic Print Society, Admail 366(A1), London SW1H 9EP (Fax: 0171-244 8999). Catalogue costs pounds 5: cheques made out to "EPS"Reuse content