Wu warned that sexual frustration might lead to social problems such as rape and prostitution.

Wu suggests that the government should prompt factories to give migrant workers regular paid holidays to return home and allow spouses to visit. Employers and authorities should also help and encourage workers to enjoy a more healthy existence outside of work.

Yin and He both added that to reduce sexual crimes by migrant workers, local governments needed to improve workers' sense of the law and create conditions for them to lead a normal life.

A good example of how to tackle this problem can be found in Nanjing's Xiaguan District. With large numbers of migrant workers from Anhui and Henan provinces, the district has built dozens of low-rent "love nests" that migrants can rent for 300 (US$37.5) yuan per month when their spouses come to visit.

"The house gives me much hope," said a construction worker surnamed Wu. "When the summer holiday begins, my wife and daughter can come and live with me for two months. It makes me happy to reunite my family in this city."