The permissive outlook of some experts on teaching children in schools about sex has sparked a fiery debate about how far such education should go.

As the new semester starts across the country, education authorities in Beijing say they will introduce the first sex education textbook for primary school students.

Education authorities say the pilot textbook, The Footsteps of Growing Up, which includes sections with titles such as "My body" and "How to prevent AIDS", aims to give pupils a basic knowledge of human reproduction.

But a section titled "Where do I come from?" includes illustrations of sexual intercourse, and these have triggered fierce debate about whether they are too explicit.

Kong Qingdong, a professor of Chinese language and literature at Peking University, goes so far as to say that such education promotes sexual perversion and crime.

In a TV interview, Kong, who is often outspoken on social issues, said: "Our sex education is way ahead of what it was in ancient times. Do we really know about sex? Is the rate of sex crime dropping at all? I don't think so. The way we look at it now, in ancient times there was no sex education at all. Adults did not talk about sex to their children. But there was no harm in that, none at all."

When people grow up they "just know" about sex, he says.

"It's like eating and going to the toilet - it's animal instinct. Besides, there are many other informal ways of learning about sex. People ... can learn by themselves in their own way. If we kill the mystery of everything, life becomes so boring."

In Japan, sex education begins early, he says, and Japanese know about every detail of sex when they are very young. "But what happens then? Ten to 20 percent of adolescents in Japan are impotent and feel no sex drive, because they have no curiosity about sex any more.

As Beijing schools prepare to introduce their first primary-school textbook, Shanghai is piloting its first sex education textbook for students in 18 primary schools in the new semester.

Compared with the Beijing book, the Shanghai version is much less explicit, with illustrations that are less graphic and with more metaphors that explain sex, such as explaining the process of fertilization by talking of sperm in a swimming race.

But Gu Jun, a sociology professor at Shanghai University, who advocates sex education being started as early as possible, believes pupils should be spared the metaphors.

"We often see that parents are too shy in explaining sex to their children, and they like to use metaphors. This simply illustrates their inhibitions."

The earlier children are taught about sex, the less impact the knowledge will have on them, he says. "There will be less negative effect on them because they will already have taken in all kinds of information about sex."

But many parents feel that teaching sex to children is wrong and harms them.

"Children are too young to learn it," says Chen Min, a mother of a boy in the second grade. "Graphic illustrations may lead to the early onset of puberty among kids."

For many years, sections on sex have appeared in junior middle school textbooks only, and the material has often been skipped by teachers.

One of the authors of the Beijing textbook, Lu Weihong, says that in China, sex education often faces resistance, especially from older generations who used to tell their children that babies were picked up in streets or jumped out from rocks.

"The children's world is pure and cannot be judged through adult eyes," Xinhua News Agency quoted Lu as saying.

China's sex education is considered conservative and schools usually try to avoid the subject, leaving the responsibility to parents or society, she says.

According to an online survey by the website Sina.com, about 64 percent of 18,000 netizens support the Beijing textbook and say it will be effective in educating children about sex. About 30 percent say the book is unsuitable for children because of its graphic sex illustrations.

In New York last month, education authorities announced that public schools will have mandatory sex education classes for all middle school and high school students this semester.

Material covered will include the proper age for sexual activity, the proper use of condoms and ways to resist unwelcome sexual advances.

As with the textbooks in China, there has been heated debate on whether the New York classes are appropriate.