I don't think it had anything to do with her gender.

I think for him he developed a bond and trust with "Ping" so when she wasn't who he thought she was it was almost insulting his trust. Or he could have been pissed that he had let her go home unharmed and here she is showing her face to everyone like nothing just happended

^yeah, me neither. It doesn't make sense. I always wondered if he did, but I don't think so. She ultimately lied to him, and that's why she lost his trust.

Exactly as tiagih said.

I agree with you but I think also that Shang can't believed her because all those people were around them and when woman's dressing as man was so big crime in this culture and there were thought that womens are worthless. But I guess that the fact that Mulan was betrayed his trust was reason one.

Shang is not completely non-sexist.
In my opinion neither of this reasons is good. Both make him narrow-minded and uncompassionate in my opinion. Mulan had very good reason to tell untruth and she is not obligated go carry of him more than of her own family. "Beatrayed his trust" are wrong words. Telling untruth is not always bad, not always something you deserve to be judge for. Only narrow-minded and uncompassionate can believe otherwise IMO.