Swinger parties began to emerge on the mainland in the 1990s as the young generation was in full embrace of Western culture to go with its new-found wealth. XFCun.com, an online bulletin board for swingers which is now 'suspended to undergo maintenance', recorded 360,000 sign-ups at one time.

However, sex largely remains a taboo topic.

District prosecutors charged Ma, 53, with having organised and participated in group sex parties on 18 occasions between the summer of 2007 and August last year.

He was reported to have set up group forums through QQ, a hugely popular instant messenger service, to recruit couple for orgies.

Ma, who has been on parole to care for his mentally ill mother but has been under house arrest since September last year, was forced to resign from his teaching job when the media began prying into his private life.

One of his lawyers, Yao Yongan , said yesterday that the court had not set a date for the verdict after the first day of a closed-door trial.

As mainland media reports have been largely concentrating on the lurid details of Ma's sexual adventures, portraying him as a disgraced professor and a pervert, sociologists and legal professors have fiercely argued in his favour.

Beijing Forestry University sexologist Dr Fang Gang said how people approached sex was one of their basic human rights.

'A progressive society should not get in the way of individuals going about their own sex lives,' Fang said.

'Group licentiousness' is a crime under the criminal code, revised in 1997, but few have been charged.

A police officer from Liquan county in Shaanxi triggered an uproar after going public in October 2006, doing an online interview over how she and her husband tried group sex and came to terms with the trauma afterwards.

She was fired by the county police but no charges were filed.

In a sweeping crackdown against vice and vulgarity since last year, the authorities ordered thousands of pornographic sites closed and proposed that computer manufacturers and internet service providers install software to filter sex-related content, although the government retracted its order under public pressure both at home and abroad.

Dr Li Yinhe , a sexologist at the Chinese Academy of Social Sciences, said she had noticed a recent upsurge in the exercise of state power at the expense of privacy and that the trial of the Nanjing professor was a sign of conservatives gaining the upper hand.

'I think if we continue to keep silent, people might eventually have their eyes gouged for taking a peek at beautiful girls on the street,' Li said.

She said that the chances of orgy organisers being punished were very rare in many other countries, and group sex in a private place could be seen as, at most, a threat to social customs, but not a crime.

China University of Political Science and Law professor He Bing said he was in favour of erasing the crime of 'group licentiousness' from the criminal code.

He added: 'Ma's case has served as a reminder over how legislation should evolve at a time when there are changes both in the way people think and the way people behave.'