Christian saints don't makethe cover of gay magazines every day – even less so in a slick of baby oil and a pair of Calvins. But such was the case with last July'sissue of reFRESH, the saint in question being played by French policeman-turned-TV-hunk, Sebastien Moura.

Was he playing Ignatius Loyola? Francis of Assisi? Paul of Tarsus? Not quite. The only saint who really cuts it as a cover-boy is St Sebastian, that curly-haired Roman youth shot with arrows on the orders of the emperor Diocletian. Sebastian's appeal to gay men seems obvious. He was young, male, apparently unmarried and martyred by the establishment. Unlike, say, St Augustine of Hippo, he also looks good in a loincloth and tied to a tree. And never was Sebastian more winsome than in the seven versions of him painted by Guido Reni, six of which go on show at the Dulwich Picture Gallery next month.

Before we look at these, though, let'srewind for a moment. Follow me, if you will, to Rome – to the Church of San Pietro in Vincoli, not far from San Sebastiano Fuori le Mura, where the martyr's punctured remains have lain since the year 287 AD. Here, in a niche to the left, is the seventh-century mosaic of a middle-aged man, bearded and in Byzantine court dress. Given the church's name, you might take him for St Peter. You would be wrong. The saint is Sebastian, although he clearly will not see 40 again and there isn't an arrow in sight.

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What's going on? Well, Sebastian is living proof of the fact that if saints didn't exist, we would have to invent them. Thanks to the arrows, he's the one martyr in art everyone can spot. (Iconography is so unfair. Who now recognises St Stephen's stones or St Lawrence's griddle?) A twinky torso also helps. Yet, according to his hagiographer, Ambrose of Milan, Sebastian was a red-blooded captain in the Praetorian Guard, a centurion of middling years: he is the patron saint of soldiers and athletes, not hairdressers. Far from riling Diocletian by proselytising for same-sex love, he was killed for converting Romans to Christianity. And we all know where that led.

But there is worse. Not only was St Sebastian middle-aged and butch, he wasn't killed with arrows. Punctured, yes, but not killed. The perforated martyr was rescued from the stake and nursed back to health by St Irene of Rome – a woman, boys – before unwisely haranguing Diocletian for his paganism as he passed by on a litter. Unmoved by his tenacity, the emperor had Sebastian clubbed to death; his body was then dumped in Rome's sewers. Had history been less kind, he might have ended up as patron saint of poo.

How this would have affected his career as a gay coverboy we will never know. I can only recall one representation in art of St Sebastian thrown into the Cloaca Maxima, and that – by Reni's contemporary and fellow Bolognese, Lodovico Carracci – is safely tucked away in The Getty Center in Los Angeles. By contrast, there are more pictures of the arrow-filled Sebastian than there are of any other martyr I can think of, painted by everyone from Aleotti to Zick by way of Rubens, Botticelli, Titian and John Singer Sargent. The National Gallery alone has a dozen, including ones by Crivelli, Gerrit Honthorst and Luca Signorelli. And they're all of the same Sebastian, the one who ends up, eventually, on the cover of reFRESH: a paragon of male beauty, his toned body, prettily stuck with arrows, exposed to our gaze; the martyr described by Oscar Wilde – who, in French exile, took the alias "Sebastian Melmoth" – as "a lovely brown boy with crisp, clustering hair and red lips".

So how do we get from a shit-encrusted Sebastian to a blow-dried Sebastien Moura? For an answer to that, take a trip to Dulwich.

Reni's six Sebastians have never been seen together before, and it is unlikely that they ever will be again. (One comes from New Zealand, another from Puerto Rico. A seventh, in the Louvre, was deemed too fragile to travel. If you need an excuse for a weekend in Paris, here it is.) It seems extraordinary that a painter should have gone back to the same subject so often, especially over so short a period. Latest scholarship dates all seven Sebastians to the 1610s, when Guido was in his thirties and newly home from Naples.

Some of this can be put down to political nous. Bologna had been annexed by the Papal States in the 16th century, and Sebastian was the third saint of Rome. Then again, Sebastian's gender-bending may have struck a chord with Reni. According to his biographer, Carlo Cesare Malvasia, Reni "turned to marble" in the presence of female models and lived with his mother until he was 55. After her death, he refused to have women in his house or to let a woman's laundry touch his own. Unlike his contemporary, Caravaggio, he seems to have had no gay life either. Sebastian is an unmistakably male saint, but one whose martyrdom is the embodiment of female passivity. Like the Virgin, his point is that he is pierced but pure. Far from being homoerotic, Reni's Sebastians are anti-erotic – a cancelling out of sexuality by a man who seems to have liked neither men, women nor good red herring.

None of this, though, explains the saint's transformation in painting from a Byzantine daddy to a Baroque twink. Here, Reni was only a follower of fashion. Piero della Francesca's Misericordia polyptych, painted two centuries earlier, already shows Sebastian as young, willowy and lightly rouged. But why?

In 1348, Europe had been ravaged by the Black Death: up to half of the entire population of the continent died in a torment of bloody flux. In their terror, Romans prayed to Sebastian – he'd survived those arrows, after all – and the epidemic lifted. Willy-nilly, he became the hottest plague saint in Christ-endom. It is incumbent upon plague saints to look as though they haven't got one foot in the grave (or, come to that, in the sewer). So by the end of the 14th century, the middle-aged Sebastian had had a makeover, his beard, wrinkles and actual cause of death neatly airbrushed from the picture.

Even so, it is something of a leap from the canvases of Reni to the cover of reFRESH magazine. Obvious answers to the question of just why Sebastian should have spent the past 400 years as gay saint du jour don't seem to add up. There are as many explanations for his appeal as there are people doing the explaining.

To Yukio Mishima, the Japanese writer and keen sado-masochist, his martyrdom symbolised the erotic pleasure of pain. In his autobiographical Confessions of a Mask, the lightly disguised author has his first ejaculation over a reproduction of a Reni Sebastian. (Just which, it is hard to say. Guess for yourselves.) Mishima later had himself photographed as the saint before ritually disembowelling himself. Derek Jarman's 1976 film Sebastiane uses the loin-clothed youth to look at the overlap between sexual and spiritual ecstasy, while Oscar Wilde and Tennessee Williams see him as a late-antique rentboy.

Perhaps Sebastian's oddest reinvention came in Thomas Mann's Nobel Prize acceptance speech. "Grace in suffering – that is the heroism symbolised by St Sebastian," said Mann; then, warming to his theme, he added: "The image may be bold, but I am tempted to claim this heroism for the German mind and German art." The date was 1929. A decade later, German gays such as Mann were being rounded up and gassed.

All of which is to say that the secret of Sebastian's success may lie in his ability to be all things to all men. Along with the famous arrows, the symbol of his martyrdom is the rope that binds his hands; yet the shape-shifting Sebastian just won't be tied down. The novelist and political activist Susan Sontag pointed out that his face never registers the agonies of his body, that his beauty and his pain are eternally divorced from each other. This made him proof against plague in 1348, and, in these ungodly times, it still does. A recent book devoted to the martyr includes Aids-related work by artists including Wolfgang Tillmans and Louise Bourgeois. It is called Saint Sebastian: A Splendid Readiness for Death.

'The Agony and the Ecstasy: Guido Reni's Saint Sebastians' is at the Dulwich Picture Gallery, London SE21, 020 8693 5254, until 11 May