Every society in human history celebrates decadence right before it collapses, and the western civilization is no different.
Looks like you're talking about homosexuality when you say decadence, which is a bit weird, because the greeks famously did the butt-stuff for much of their existence, and it doesn't seem to have changed much. The theban sacred band, an elite group that broke the Spartans, is probably the best-known example in pop knowledge.
We could turn to Rome, whose commentators and politicians were complaining about decadence around 200 b.c. Rome collapsed nearly 700 years later.
You haven't identied what "celebrating decadence" would be, but Id put it at maybe around 50-200 ce. You know, Caligula and Nero, with their absurdly debauched banquets and indiscriminate fucking. And Trajan's 123 day victory festival, or Commodus's ridiculous games. Which occurred 300 years or so before the collapse.
At which point we could turn to the Eastern Roman Empire. The Byzantines were famous for decadence-the name is a byword for the practice. Theodora, the most powerful person in Europe in the Sixth Century, was a former prostitute who seized the emperor's eye. They enjoyed a reputation for decadence, sodomy, gluttony, incest, etc for the rest of their existence, which lasted another 900 years until Constantinople fell to the Ottoman Empire.
Led, of course, by Mehmed the Conquerer who had a harem, five wives, and, topically to this thread, at least one young man in his harem, and another friend/hostage who he kissed on the mouth and invited to bed (Radu Cel was Vlad Dracula's brother. Really cool stuff.) Despite these LGBT bonafides, he and his successors terrified Europe for the next hundred years or so. The Ottoman Empire itself survived until the 20th century, at which point it became the modern republic of turkey.