I think sleeping with certain students is ok: like postgraduates or fourth year undrgraduates who are a bit more mature maybe.

But what would your employer think? And would he take steps against you? In China, it's all too easy for employers to dismiss FTs for a concocted reason. Instead of using the sleeping with a student reason, he could simply say your students are dissatified with your teaching. Being dismissed during the semester will entail financial losses such as no air fare allowance/reimbursement being paid to you.

Anyway, follow your own conscience, and good luck!