During her first survey in 1989, 15 percent of the 2,500 young people in Beijing she interviewed reported having premarital sex, and most of the outliers were already engaged and simply waiting for the bureaucracy to produce a marriage license.

In a nationwide follow-up study with 4,000 subjects she did in 2013, that figure had increased to more than 70 percent.

“The changes have been revolutionary,” she said.

Still, the government is priggish when it comes to matters of sexuality. Communist Party members can be purged for serial infidelity, orgies are strictly illegal and television censors have been on an anti-cleavage campaign of late, though their efforts have generated widespread public ridicule. One of China’s biggest online pornography operators is serving a life sentence.

Professor Li practically harrumphs when asked about the government’s antisex policies. “Medieval,” she says with a roll of the eyes. She does more than complain. In 2010, after the police arrested 22 members of a swingers’ club in Nanjing, she was one of the few public figures to speak out in their defense, calling the charges a violation of basic human rights.

The so-called group licentiousness law provides up to five years in prison for consenting adults who repeatedly have sex with more than two other people. “Laws like this need to be abolished because the purported crimes don’t have any victims,” she said.

Still, she considers it progress that the one defendant who refused to plead guilty, a middle-aged computer science professor, was given three and a half years in prison. Last year, a doctoral student in Shanghai was sentenced to just five months for a similar crime, this time involving several gay men who met one another online.