That defense and the investigation, which is facing a critical judicial hearing in late November, have offered a keyhole view into a clandestine practice in certain powerful circles of French society: secret soirees with lawyers, judges, police officials, journalists and musicians that start with a fine meal and end with naked guests and public sex with multiple partners.

In France, “Libertinage” has a long history in the culture, dating from a 16th-century religious sect of libertines. But the most perplexing question in the Strauss-Kahn affair is how a career politician with ambition to lead one of Europe’s most powerful nations was blinded to the possibility that his zest for sex parties could present a liability, or risk blackmail.

The exclusive orgies called “parties fines” — lavish Champagne affairs costing around $13,000 each — were organized as a roving international circuit from Paris to Washington by businessmen seeking to ingratiate themselves with Mr. Strauss-Kahn. Some of that money, according to a lawyer for the main host, ultimately paid for prostitutes because of a shortage of women at the mixed soirees orchestrated largely for the benefit of Mr. Strauss-Kahn, who sometimes sought sex with three or four women.