Extreme caution should always be used. You might avoid Aids but you can get other nasty stuff like the clap. I knew one French man who contracted syphilis in China. SOme cities in China now have a 72 hours visa that are given without medical checks, or even just a plain tourist visa that people use to go partake of the locals.
I hear you about those bar flies. Sometimes you just have to say no and go home alone. This goes with what I tell new teachers: "Say no more than you say yes."