Just a thought,..but while you have found a key trigger, for your husband to work on to help you feel better, you might want to re-examine your thoughts towards flirting.
Even in a monogamous world, in many types of social circles, monogamy does not mean 'dead'. It simply means there is a line, you do not cross. In many circles flirting is fun, and complimentary. As long as everyone knows who's bed they are obligated to each night, that defines the 'taken' aspect in many monogamous lives.
Soooo to answer the question of why they are hitting on a man they think is married and mongogamous,... it's not to disrepect you, but because it feels 'safe' to flirt with someone already 'taken'.
Of course there is always the ready-and-willing kind, but that's their problem, not yours,..jealousy and anger, make it your problem.
I know,..easy to say, harder to do.