I wasn't wearing anything particularly sexy—just a black bikini top and bottoms. Before I left home, I pulled on a pair of wide-leg leggings, a basic black tee, and a faux-leather trench—an outfit meant to blend in with both the sea of city commuters and the crowd I was heading toward. I took the subway to lower Manhattan, double-checked the address, then hopped into an elevator with about a dozen strangers. At the coat check, we all disrobed. And then, I stepped into my 10th sex party.