A population the size of China is going to have plenty of people that don't fit the M/F choice. For an extreme example a fully functional hermaphrodite could be both or self identify as one or the other. Some people are XXY genotype or even XXXY so even genetic testing can be ambiguous.
There mostly male, but often with significant female secondary sexual characteristics and sterility. This is of-course ignoring Mosaic people which have two set's of DNA so you can be XXY and XX at the same time...
http://en.wikipedia.org/wiki/Mosaic_(genetics)
http://en.wikipedia.org/wiki/Intersex