The sexual underworld of NYC has always held a certain intrigue. In this huge, dynamic, crazy town, there have always been sex clubs and swingers parties, orgies held in hotels and apartments, as well as good old fashioned prostitution rings (remember the Mayflower Madam?) and people hooking up in bars. Any proclivity was catered to. There used to be legendary bath houses like Plato's Retreat, plus gay bars like Stonewall and dance halls like the Roxy -- and there were also places like S&M "dungeons" and other creepy places where you could lie under glass and have people ... well, let's go no further down that road.
You can't have 8 million+ citizens and not have some kinky people in it.
But the composition of sexy NYC is changing. No, it's not that New Yorkers are any less horny than today, but the sexual life is the city is getting less, well, seamy. The gay clubs and dance halls are vanishing. Now gay New Yorkers are hooking up online. Sex clubs are getting busted. Gentrification and "niche" has also played a part.
In short, the sexual life in NYC is getting more introverted, less extroverted. And, as the 21st century grinds on, this will probably continue.