The radicalism has gone, but the poignancy remains. In 1968, a whole year before Stonewall, Mart Crowley put gay men on the stage. A classic boulevard comedy, The Boys in the Band slotted their lives into mainstream entertainment. Homosexuality was still illegal outside of Illinois when it landed off-Broadway, and the sort of parties it showed, a birthday bash for a group of gay friends, took place behind closed doors. Putting them out in public, Crowley outed the personal pressures and struggles faced by the gay community. All of those present are battling to be themselves.

None more so than Michael (Ian Hallard), its bright, but bitter, host - uncomfortable in his skin and his sexuality. He throws open his doors to a full spectrum of homosexuals, from James Holmes' flouncing Emory to Nathan Nolan's straight-acting Hank, and here, in private, they find some form of freedom – dancing, joking and swapping shaggy-shag stories. The arrival of Michael's old college roommate Alan – the only straight in East Village – punctures the party. Black-tied and basso-voiced, John Hopkins's face can't mask his horror. It's as if Clark Gabel found himself dining with cannibals.

Beneath their party faces, there's a swirl of shame and secrecy. What starts as a shared joke – the awkwardness of what Michael calls ‘Christ I was drunk syndrome' – gradually wears thin until the sadness shows through. It's there in Donald's shyness – Daniel Boys is eloquently tentative – and in birthday boy Howard's snarky defence mechanism. Mark Gatiss, one hand in his velvet pocket, a look of permanent distaste on his acne-scarred face, is superb: the arch queer with a sneer.

Hallard suggests that Michael tries to match him, as if he's never truly found an identity of his own, and as his acid levels rise, you notice that he's swapped soda for scotch. Self-loathing leads to self-destruction and he forces a callous game on his guests: Affairs of the Heart – a kind of Russian Roulette with unrequited love. Each must phone the man they most love and confess. You glimpse the happiness that each missed out on.

Crowley's writing does show its age: a little too classical, a little too contrived. The need to show every side of homosexuality – from camp to closeted – leaves too many characters lolling around a living room waiting their turn, and the comic relief – Jack Derges' dim stripper in a cowboy hat – can be clumsy. Behind that, though, is a poignant expression of inhibited self-expression.

Adam Penford's revival raises a bigger question. Where are the contemporary equivalents? When was the last time we saw a major new gay play? Gay couples, yes, but gay lives, no – not as lived in this country. The last I can think of is DV8's John – and that had to be sprung on a National Theatre audience by stealth. As assimilation's occurred, new writing theatres have turned their back on gay issues. Mostly, these days, they're confined to the fringe: chemsex at the King's Head, homophobia at the Old Red Lion. Gay issues haven't gone away, but, in theatre, they're largely going unseen. That makes Crowley's 48-year-old play more radical than it should be.

The Boys in the Band runs at the Park Theatre until 30 October.