A student of mine once told me that his favourite film – or, as he put it, "the best film ever" – was Sliding Doors, in which John Hannah woos Gwyneth Paltrow by repeating Monty Python sketches. It was made in 1998. On hearing this, I knew immediately that the student was heterosexual. It could not be otherwise. My favourite films, on the other hand, are The Leopard, West Side Story, Celine and Julie Go Boating, A Touch of Mink, Even Dwarfs Started Small and the bit in Dil Se where 50 dancers do the cancan on the roof of a moving train.

I don't think anyone, hearing this list, would be under any illusions.

What does it mean to be gay? Is it enough, as many people think, just to fall into the sex-clinic's category of "men who have sex with men"? That is intended to include the closet case and the cottager who goes home to wife and children. There are plenty of people – increasing numbers, in fact – who are gay without having much to do with traditional gay culture. There are gay people who follow rugby and even play it – not necessarily in a pervy way – and those who genuinely quite like the New Statesman. Some gay men live their entire lives kitted out in beige anoraks. Some of them collect stamps and others work for engineering companies. Some of those men – gay but not Gay, as it were – regard the whole musicals-interior decoration-fashion-thing as a curious foreign language, not really worth learning. They have never said "Bona" or "fabulous" in their lives; the only musical they have ever seen is Phantom of the Opera, because their aunt took them. What their culture is, and whether it forms a unity, the cultural critic cannot, apparently, say. What he can be concerned about, it seems, is the culture of Gay, passed down through generations of slappers, propping up the bars of Soho in London, Chelsea in New York, and the Marais in Paris, all quarters which are now as dead as the proverbial dodo.

David Halperin has written an over-long book, more localised in its application than he seems fully to appreciate, about the aspects of being gay other than sexual choice. His thinking arrives courtesy of a course he teaches at an American university. Naturally, when a course in "How to Be Gay" was announced in the American mid-west, an army of enraged family-first campaigners rose up in taupe leisurewear to denounce Professor Halperin for wanting to recruit the innocent. The passages recounting this provide the most amusing sections of the book, as taupe leisurewear and its mental equivalents so often do. He admits that "American" is an unspoken adjective in much of what he has to say, including the title of the book – I guess "How to be an American Gay" would be an even more uninviting subject than the one he has chosen. Outside America, he reliably gets things wrong, suggesting that Bollywood musicals may represent the same sort of gay cult to Indian gay men that Sex and the City does to Americans – he's clearly never seen a film in a Calcutta cinema, or he would have noticed that the appeal is not a gay thing at all at its source. He's not even very good on opera, bringing up Aida as his prime example – if he knew any opera queens, he'd know that we are much more likely to be going to Tristan, Salome and Janacek. When I last saw The Makropoulos Case, the stalls were like Lo-Profile on a Friday night, packed with queens waving at each other, opera glasses in hand.

In fact, the limits of the book are set not just by the limits of his culture, but by his understanding of what culture might be, even just in America. His interests are not really in gay culture at all, but in gay taste, particularly in film and TV shows. He doesn't show much interest in gay meeting places – when he does record finding himself in a backroom, it is to talk about the porn playing to an accompaniment of 19th-century opera. He doesn't, amazingly, show any interest in clubbing, which I would say was a much more powerful expression of gay culture to recent generations of gay people than terrible old movies. He doesn't write about clothes, or gestures, or gait, or any of what identifies a gay man to another at 80 paces, or the syntax and vocabulary and slang which makes them mutually clear at closer quarters – I mean, you can't always be saying "Have you seen Mommie Dearest?" to strangers. Halperin pretends to be an outsider looking in, but you only need to look at the Earls Court, circa 1985 moustache ornamenting his face to realise that he's writing from well within his own culture, looking out, but not looking very far. Perhaps if you stand still long enough, you become an outsider, as the culture moves swiftly on, from Judy to Gaga.

Those old movies are the backbone of the book, and especially ones starring, or about Joan Crawford. The book would have been much stronger if it were a disciplined 120 pages about Mildred Pierce, and its off-shoots – Mommie Dearest and various drag-show reconstructions – specifically examined from the viewpoint of a middle-aged gay man. That is not a subject without its interests, but Halperin's attempts to shift Crawford into the centre of what gay culture means and meant is plain old weird. No wonder many gay men feel left out when they read accounts like this, and wonder what they look like, having no interest in shopping for soft furnishings, Dusty Springfield, saying "Miss Thing!" or the perfect white shirt. I feel pretty much left out, never having seen Mildred Pierce, as it happens.

In a blissfully funny book about homosexuality in the 19th century, Strangers, Graham Robb provided a list of things that at the time were thought to cause or to indicate homosexuality. His list included a lack of physical exercise, or on the other hand excessive riding of horses; too much meat-eating, or possibly anaemia; impotence or sexual overindulgence; plebeian brutishness or aristocratic refinement; too many available women leading to satiety or too few leading to lack of opportunity; lack of parental love or excess of same; celibacy or marriage. It was surprising, in fact, that anyone ended up heterosexual, so all-encompassing were the causes.

Among the qualities Halperin identifies with gayness are being "a great dancer or cook, that you have a weakness for mid-century modern, or that you drive a VW Golf" – a VW Golf? Really? Elsewhere, lists of gay-connected activities can stretch to surprising places: "being gay had something to do with liking Broadway musicals, or listening to show tunes or torch songs or Judy Garland, or playing the piano, wearing fluffy sweaters, drinking cocktails, smoking cigarettes and calling each other 'girlfriend'." Playing the piano? Smoking cigarettes? Some cocktails, I can tell you, are more gay than others – the Cosmopolitan very much so, the Negroni, not so much. Elsewhere, Halperin wonders whether gay culture is dying out, on the basis that – he claims – straight people bought up the houses of gay people after they died of Aids in the 1980s and 90s, and that the numbers of gay bars in major cities are on the decline from peaks in the 70s. He blames online hook-ups, which seems quite plausible. But is this really evidence of gay culture dying out? After all, those millions hooking up on Grindr are forming powerful connections with hundreds, sometimes thousands of other gay men, tossing each other aside afterwards with savage abandon, and are doing it without the aid of commercial bars. That doesn't seem like such an awful thing.

The age of the camp persona, the biting comment, the ironic allusion is not passing, exactly, but it is moving towards a particular section of a particular gay community at a particular point in time. It would be good to know what proportion of gay men identify with that. An interesting book about gay culture would spend time with a range of gay men, of different ages and classes and backgrounds, finding out how their social networks were formed, as well as investigating how they liked to be entertained.

Of course, that would be a great deal harder than just going on about a couple of scenes from 40s movies that you and your friends simply adore. You'd hardly guess, from Halperin's account, that gay culture was anything but owned and demonstrated by late-middle-aged white men in about four American cities.

This cultural confidence is driven on by its usual fuel, money – some time soon the crucial gay diva will not be Lady Gaga, but Fish Leong. And there are 18-year-old black boys within a mile of David Halperin who have never heard of Joan Crawford. They have their culture too, and their own ideas, hugged however secretly to their hearts, of how to be gay. If they ever read this book, which they won't, they would probably put their hands to their throat in the gesture known to German gays as "the necklace of pearls", roll their eyes, and say what we always say of a tragic effort all round – "Pur-lease."

• Philip Hensher's Scenes from Early Life is published by 4th Estate.