Happened to my brother all the time when he was there. He's half asian, but has a fairer complexion and hair color, is 6' tall and was probably rocking a beard at the time. He was never a dick about it, though. Imagine going your whole life and never seeing a white guy in person; you might be excused for gawking a little when it actually happen. Mainland Chinese are notorious for lack of manners by western standards. From what I hear, black people have it the worst when they visit China.
My Caucasian mom was visiting Taiwan and some random mainland Chinese women wanted to take a picture of her. She thought it was strange but she went with it. She asked my brother what that was all about and he said that they were taking pictures with her because she has big tits and they were going to show it to their friends. She felt pretty upset about that but my brother told her is normal.