Gay Icons, National Portrait Gallery, London

An exhibition of icons chosen by 10 famous gay curators shows that one person's hero is another's turnip head

As anyone familiar with the work of the lazy journalist will know, "icon" has become one of the most devalued words in the English lexicon – not least in its gay variety.

Once, a gay icon was equated with a particular kind of big-lunged, big-gestured, tragedy-tinged female performer ... Judy, Liza, Dusty et al. If yesteryear's conception of gay iconography was narrow, however, today's is simply tawdry. Now every popstrel who has graced the stage at G.A.Y., or ex-boyband member willing to flash his pecs to stay on the D-list is chasing the accolade. In this context, any attempt to rescue the term from the dregs of popular culture, to redefine what actually makes a gay icon iconic, should be applauded.

Sadly, the most impressive aspect of this Gay Icons exhibition is Richard Dyer's lucid essay, "The Idea of a Gay Icon", in the accompanying book. By comparison, the exhibition is muddled. What we get is 10 famous gay curators making personal choices. The idea is to reflect what a diverse bunch we are, each with our own backgrounds and inspirations. It's the word "icon" that's the issue: isn't an icon, as Dyer points out, necessarily representative of a "wider category"? Shouldn't a gay icon speak to some communal sense of gayness?

This flawed approach is most evident in the case of Elton John. He has picked "my lyricist" Bernie Taupin, "my mentor" Gianni Versace, John Lennon, "a huge inspiration to me", and, most ludicrously, football manager Graham Taylor, his old employee at Watford FC. These selections tell us nothing about gay experience, and everything about Sir Elton's ego. Meanwhile Billie Jean King includes her family and Nelson Mandela, selections as banal and unilluminating as the accompanying text. "My family shaped and guided me to be the person I am today," she tells us, at which point you imagine you've been transported on to the set of some gruesomely gushing talkshow.

However, enough of the gripes. Elsewhere, Ben Summerskill expertly weaves humour and gravity, the personal and the political, into descriptions of Martina Navratilova, Ellen DeGeneres, and Joe Orton among others. Sarah Waters, Jackie Kay, and Alan Hollinghurst may have you speeding to the bookshop after lauding some lesser-known gay authors. Chris Smith strikes a hauntingly elegiac note with his selections of codebreaker Alan Turing and mountaineer John Menlove Edwards, both of whom killed themselves. And good on Waheed Ali, whose picks include The Village People, Lily Savage, and porn star Jeff Stryker and isn't afraid to state the obvious and remind us – in an exhibition which at times feels rather too staid – that "sex has been, and always will be a defining aspect to sexuality".

As far as the portraits go, a few stand out. Savage – headscarved up, fag in mouth, and perched in front of Blackpool Tower – a peculiar strain of British camp incarnate; Harvey Milk addressing a crowd in the year he was assassinated, serene defiance spread across his face; and Stryker, luxuriating in bed, a symbol of unfettered male sexuality at once lascivious, romantic and absurd. There's a rich narrative of triumph and tragedy, pride and shame, glamour and irreverence lurking inside this exhibition; it's just a shame it comes mired in bittiness and irrelevance.