PROFESSOR Liu Dalin's early research focused on marriage. He read a lot of books about China's sex culture written by foreign scholars but found very few that were written by Chinese themselves.

"Western scholars' research is rich in material and I admired their writings a lot. I felt it strange that there was so little in Chinese," Liu said.

When he decided to turn his attention to writing about sex, a colleague at Shanghai University said: "Your research on marriage is so good, why change to researching sex? I am worried it could put you in some danger."

Actually, Liu didn't run into any danger, but he had to overcome many obstacles. Once his wife complained that her colleagues were joking that: "We see your husband is writing essays about sex again in magazines." She said this made her feel shy and embarrassed.

However, Liu didn't stop writing about sex although to "protect" his family members, he used different pen names to avoid being recognized.

Liu said food and sex are the two essentials for human beings.

"I don't know why food can be thought of as part of culture but sex is a topic that can't be talked about publicly," he said, sounding a little annoyed.

At the gate of his museum there is a quotation from Engels: "In the end, at least German workers will become used to talking freely about what they do day or night, talk about the natural, essential and ordinary things ..."

Liu said: "But now, most people think sex is an embarrassing subject to speak about. It is a serious social problem."

A survey in China has found that 44 per cent of urban families and 33 per cent of village families are not satisfied with their sex lives. One reason given was that they did not understand the function of sex very well and did not know how to improve their sex lives in a scientific way.

People's traditional concepts placed obstacles in the way of Liu's research. He still remembers a crackdown on pornography by the city police in 1990 when some ancient paintings depicting sex scenes were condemned as phonographic. Although Liu explained their value, they were still burned.

In 1987, a publishing house in East China's Shandong Province printed 40,000 books of Liu's book, "Sexology". But before it was launched, the authorities banned the book because they were alarmed at the high public demand for it. However, after a year, the book was released for general sale.

"Before doing my research, I had thought that people living in the Song (960-1279), Ming (1368-1644) and Qing (1644-1911) dynasties had even more stick-in-the-mud attitudes than people today," he said. "But I found I was wrong."

After collecting many ancient examples of literature about sex from around the country, Liu concluded that the common people of earlier times were actually very open although the authorities imposed many restrictions on them.

"On the surface, the people of ancient China were very conservative. But, in fact, they were more open than we are. That is because people's natural appetites can't be destroyed," he said. "Sex is one thing that can't be repressed.

"My last wish in my life," said the 71-year-old academic, "is to set up a fund, if my museum can make a profit, in order to pass the museum on from generation to generation."