In this video, Laura Ling took a close look into the sex industry in Beijing. She says that "most sex workers are migrants, part of the estimated 120 to 200 million people who have left China's countryside in search of better-paying jobs in China's cities." [Click to see the origial video]
Also related, First Sex Report On Farmer Workers Published, from China.org.cn:
Zhang Wenwei, Vice Director of the Shaoxing Health Bureau in Zhejiang and also an expert on reproductive medicine, designed the questionnaire.
The survey interviewed 776 farmer workers in Zhejiang, with 481 of them married -- 307 males and 174 females. [Full Text]