It's an unfortunate fact of life here in Thailand- the sight of a Western man with a Thai woman (even a very young one) equals "prostitution" in many people's eyes.
I've had issues when eating out with my wife- waitstaff has treated her poorly and used improper honorifics when speaking to her- more than one restaurant manager has found himself taking some very harsh criticism from me over that point.
Even after a decade here it still bothers me (even though the reason for people jumping to that conclusion is obvious).