Maybe I should move to China

You can try to stigmatize it if you want, but people are going to have sex, often outside the bounds of their marriage.  Like life, sex will find a way.  And the more you try to prevent it, the more deviant it will become as it is pushed into the hidden corners of society.  So no, I'm not surprised that mistresses are common in China, nor do I think it should be labeled an "epidemic," implying without basis that it is a problem that needs to be corrected.