Well, as others have said before, it's certainly not immoral, but as far as professional ethics are concerned, yes, I'd say a teacher who dates with and sleeps with his or her own students is professionally unethical. I can understand though why some foreign teachers and Chinese teachers also are tempted to sleep with their students: Some young Chinese women are pretty hot. Therefore it's better not to put yourself if temptations way. That means to never invite your own students to your apartment, as one thing leads to another.

If it is a former student and you know for sure that she will never be your student again, well, then it does not matter, you can be lovers. I also think it's better not to look for a student at your own university. If you want a Chinese GF it's better to find one from a different university.