I do not think there is anything that you can do to make him see what he already knows. I think he is full of crap if he says that he sees nothing wrong with it. I think nothing happened......yet. But he knows he is putting himself in a position where something may happen. That is why he never told you about the lunch or the woman before. He probably told you after because it just did not feel right.
Your husband states that he can have lunch with anyone as long as he does not cross boundaries? I feel having lunch crossed the boundary. Married people should not put themselves in positions that they can start to fall for someone else.
Maybe you can tell him married people should not put themselves in positions where they may eventually cross the line. If you see some attractive guy on the street and you start talking to him,,,,,then you start having coffee dates, then lunch dates, then movie and dinner dates, and you are getting to know him more and more and sharing more and more..... one day a boundary will probably be crossed. So the whole point is not to every put yourself in that position to start with.
both husband and i have friends of the opposite sex. in none of the instances are they people we met at starbucks.
work, school, reading groups, running clubs...etc...not starbucks.