Prosecutors in the east Chinese city of Nanjing have charged 22 alleged members of a wife-swapping internet chat room with "group licentiousness".
A university academic, company bosses and shop assistants are among 14 men and eight women facing up to five years in prison if convicted.
Group sex parties were reportedly held at the academic's home.
The trial has sparked debate between conservatives and liberals, a Beijing-based blogger told the BBC.
Stan Abrams, who writes on Chinese legal matters in his China Hearsay blog, said prosecutors were using a little-known article of criminal law against the defendants.
The 22 defendants are accused of engaging in dozens of group sex encounters between 2007 and 2009, according to China's Procuratorial Daily newspaper.
Ma Xiaohai, a 53-year-old associate professor at an unnamed university in Nanjing, was charged with setting up the chat room and organising group sex parties at his home.
"At first the chat room discussions were very clean, with most people discussing their marital problems," the paper quoted him as saying.
But "swinging" became the focus of the forum, which grew to include more than 190 members, the paper said.
"Every family more or less has this or that kind of insufficiency - a marriage can be like a bowl of cold water that has to be drunk, swapping partners is like a bowl of sweet wine," Mr Ma was quoted as saying.
"You have two camps that have come out on this trial," Mr Abrams told the BBC World Service.
"On the one side, you have got the conservatives and, frankly I think, the government, who are saying there are public policy issues here, there is the matter of social order to think about.
"On the other hand you have people who want liberalisation, who want the law to reflect the reality of the situation, who are saying that not only there should be legal reform, but the government should stop cracking down on these people for these kinds of activity."