Derek Jarman: Brutal Beauty, Serpentine Gallery, London

When the artist's homosexuality is integral to his work, he risks being left behind by a changing moral climate

I wondered, writing this, whether it would be possible to review a show of Derek Jarman's work without using the word "gay". It niggled, too, to think what it might mean to do so – for me as a gay man and critic, for Jarman as an artist, for the notion of gay art. The last was a genre whose existence Jarman endorsed. But does pigeonholing him as a gay artist get in the way of seeing him as an artist? Conversely, would Jarman hold up as an artist were his work not prefixed by the word "gay"?

Derek Jarman was diagnosed as HIV positive in 1986, and the Aids he developed took seven dreadful years to kill him. Apart from a handful of clips from his films of the 1970s, playing silently in the Serpentine's octagon, the works in this show all date from the years between his diagnosis and his death in February 1994 at the age of 52. This places them in a time when being gay and being ill were regarded as synonymous in the public mind, an elision memorably summed up, in the month of Jarman's own diagnosis, when the Chief Constable of Greater Manchester described homosexual men as "swirling round in a human cesspit of their own making".

It wasn't a time for sitting on the fence, and Jarman, who made his HIV status public within a month of knowing it, didn't. His work post-1986 was uncompromising, in-your-face, shorn of the lushness of his best film, Sebastiane (1976). The Roman martyr's story had been a trope for the eternal persecution of gay men, Jarman included. Now, the artist dropped his metaphors and became a martyr himself, the centurion's famous arrows replaced by his own Kaposi's lesions and retinas that had been eaten away by cytomegalovirus. After Blue, no longer able to see, Jarman stopped making films and concentrated on painting, working in his cottage on the beach at Dungeness. A single work from this last period, called Fuck Me Blind, is included in this show. Its whorls and stabs howl against an artist's loss of sight, with Miltonic rage.

Mutatis mutandis, then: the work at the Serpentine was made in an era before combination therapies offered the illusion that Aids was curable, before the Civil Partnership Act allowed gays a proximity to marriage. It was necessary for someone to do what Jarman did, and he did it heroically. Isaac Julien's feature-length film Derek – the Serpentine's exhibition is curated by Julien, a friend and protégé of the artist – shows Jarman to have been humane, amiable and brave. It is impossible not to like him, perhaps (like his muse and the star of his films, Tilda Swinton) to love him.

And then there is Jarman's work. Walking around the Serpentine, I found myself thinking, unexpectedly, of the pictures of Aleksandr Deyneka, the girl-meets-tractor certainties of Socialist Realism. From 1934, the Soviet state demanded that art be clear and partisan; in the great cause of Stalinism, there was no room for ambiguity or even-handedness. And Jarman's art, too, baulks at the neutral and unclear. His suite of Bed paintings – numbers III, IV and VII are in this show – are sloganistic, the slogan in question being we're here, we're queer, get used to it. Jarman's beds may pre-empt Tracey Emin's by a decade, but they have a less subtle purpose: to provoke outrage by incorporating photographs of men sucking each other off, by using impastoed tar to evoke the shittiness of anal sex.

At a time when many gay men were lying low, Jarman was unapologetic about his sexuality. But the question you ask as you look at his Beds, as at Deyneka's Collective Farmer on a Bicycle, is whether slogans really make good art. You also ask it watching Blue, a work it is difficult not to compare with the playful monochromes of Yves Klein. Where Klein allows his ultramarine to tell its own story, Jarman tells Blue's for it. His voice, cod-Shakespearean, records his daily life as a blind man dying of Aids. It is, by turns, whimsical and portentous: the line "Blue transcends the solid geography of human limits" makes you squirm. With the exception of Fuck Me Blind, I found the works in this show laboured and obvious; as, historically, they were called upon to be, artefacts rather than artworks.

Serpentine Gallery, London W2 (020-7402 6075) to 13 April