I plan to joke with my chinese girlfriend about her having a flat chest.
This sounds more like an insult to your girlfriend over a joke.
She doesn't need to know about Chinese guy's preferences.
You shouldn't make a joke about your partner's physical appearance. They'll get upset and will think you find them unattractive. They may be offended enough to end your relationship.
It's the truth. I can't possibly love her forever if she is flat chested.
If you feel that you can not love her for this reason then you should not be with her. She deserves to be with a man that can love her. Hopefully you understand what I am saying here since you also deserve to be with someone you love.