To understand the idea properly you have to at least have a passing acquaintance with its progenitors.
London in the 1890's was, like France, a hotbed of decadence - if you knew the right people. A man of the 1890's considered himself liberated, post-Darwinian, and, bereft of purpose and meaning.
It was a struggle that captivated and determined the course of one well-known life, that of Oscar Wilde, whose novel "The Picture of Dorian Gray" was a handbook of Decadence. Except that it is, as Wilde protested, a deeply moral book, (the narrative declares of the famous painting: "a visible symbol of the degradation of sin").
Here is typical Dorian:
One evening about seven o'clock I determined to go out in search of some adventure. I felt that this gray, monstrous London of ours, with its myriads of people, its splendid sinners, and its sordid sins, as you once said, must have something in store for me. I fancied a thousand things..
Wilde set out the stall in the book as a kind of handbook of Decadence, often including lists of all the bad things one could do - in case your own imagination faltered. Wilde writes that Dorian was "poisoned by a book". That fictive book was almost certainly inspired by Huysman's Against Nature - a satire on literary realism and decadence. It outdid Wilde on several levels, especially on lists of bad things to do, including, getting a tortoise and covering it in jewels, and having a black dinner party where the food was tinted in dark colours, served by naked negresses, clad only in stockings.
Wilde, like many of us duelled with the good and bad side of human nature. His Dorian was Lord Alfred Douglas, who stuck with Wilde like a clam in the days of fame and success, and who deserted him as soon as disaster struck. Wilde was the artist, Douglas, his homosexual lover, was the parasite. (For the sake of brevity, I have to make this sweeping generalisation - Douglas had some competency as a poet.)
Wilde's downfall, the trials, the imprisonment, the ignominy and his death in a cheap Parisien hotel, are due to his succumbing to the lower tastes, indeed the lower life that Bosie offered him. (You can read all about it in De Profundis). It is not that Wilde was not thoroughly homosexual or thoroughly promiscuous, it is that Bosie distracted him from going on to further heights. When the fall took place, Wilde was, as they say today, on a roll. He had three of his plays running in London theatres. Together they were spending obscene amounts of Oscar's money, with the result that Wilde was bankrupted soon after his imprisonment for the kind of money he would spend in a week.
In a nutshell, Decadence was not so much about sins or committing them, but about staving off boredom and perhaps attempting to discover meaning through new sensations. In order to be decadent you first had to be rich enough to indulge yourself, usually because, unlike the working classes, you had free time. Decadents affected the word "Ennui". It is only now in the 21st Century that it has taken on a different meaning, and that is the pursuit of pleasure and excess for its own sake. Wilde would have been mortified. He saw decadence as a means of artistic inspiration and at worst, a distraction from the serious business of writing.
All this brings me to Stephen Gately, a young man who found some fame, if not in life, then certainly in death. I am not here to discuss the artistic merits, if any. but you may observe that in the end, as he lay motionless on the sofa, whilst his "husband" and someone they picked up at a gay bar were busy in their bedroom, that decadence, for want of a better word, ruled the day and condemned his partner, to a lifetime of regret. I am not suggesting there is a connection between cause and effect here, merely that Gately's partner may have cause to reflect upon the shallowness of his life and how, by accident, that apotheosis of indulgence and selfishness was in the ascendent in the wrong place, at the wrong time.