In the sleek white atrium of London’s Alison Jacques Gallery, there are two contrasting pictures which sum up Robert Mapplethorpe’s strange allure. One is a huge photo of a young man in a state of sexual arousal. The other is a small photo of frogs scampering across a plate.

Oddly, it’s the smaller photo which seems more shocking, more disturbing. Mapplethorpe had a talent for making even the most extreme sexual practices seem humdrum. It’s when his models put their clothes back on that things get really weird.

His models were his friends and lovers. Photography was his autobiography - he was a participant, not a voyeur

Mapplethorpe died of an AIDS-related illness in 1989, aged just 42, but after all these years his fame – and infamy – shows no sign of fading. There have been retrospectives of his work this year in Paris, Oslo and Los Angeles. There are shows running in Turin, Stockholm and Montreal.

So what is it about Mapplethorpe which transfixes us? What is it about his pictures which continues to command our gaze?

Mapplethorpe made his name with his graphic depictions of New York’s Gay community, specifically the leather bars which occupied a secret corner of that scene. His bold studies of butch macho men were revolutionary. No one had portrayed this subculture so candidly before. American was scandalised, and his celebrity (and notoriety) was assured.

Yet there was more to his art than shock and awe, or S&M. He was a perceptive portraitist with a classical flair for composition. Whether he was shooting a still life or an erect penis, he found theatricality in every scene.

“Really, what he was interested in was sculpture,” says Alison Jacques. “There’s something about the way he looks at things which is utterly unique.”

The thing that gave Mapplethorpe’s art such power was that he lived the life he photographed. His models were his friends and lovers. Photography was his autobiography – he was a participant, not a voyeur.

“Whenever you make love with someone there should be three people involved – you, the other person and the Devil,” he said. The same could be said of his portraits. There was a ruthlessness about his work, an indifference to the human cost which resembled the amorality of a sexual predator.