Concubinage has a long history in China and was common right up to the early 20th century. Emperors and warlords kept many concubines as well as wives, and the wives sometimes even presented their husbands with fresh concubines, a practice known as "drinking vinegar".
Welcome back: Return of capitalism to China means a major comeback for the concubine
http://www.independent.co.uk/news/world/asia/welcome-back-return-of-capitalism-to-china-means-a-major-comeback-for-the-concubine-1777215.html