Is it irresponsible to date a girl and not intend to marry her?
In my opinion, it is not irresponsible if it is made clear to the girl that is the situation. Then it is up to the girl whether she wants to continue dating him or not under those conditions. If he lied about his intentions, or avoided all questions about it, that would be irresponsible and just wrong.
I emphasize, I'm only giving an opinion here - and a western opinion at that. Your values may give you a totally different judgement. I won't pretend I'm an expert in relationships and certainly not Chinese relationships.