Most Helpful Guy
You need to take a hard look at this relationship. That is a lot of mistrust and anger to have. He didn't hide it from you, and it is a pretty common thing to go out for lunch with co-workers. The fact that you actually started looking at divorce options for something like this, where he was open and honest about something that seems to be completely innocent, is a huge red flag for the stability of the marriage.
Yes, I would say it's really ok that he did it, and that you are blowing things out of proportion. I think that it is harmless that he agreed to the lunch.
I think it is not ok that you are looking at 'options' because of this. And I do not see the marriage lasting though actual hard times if this kind of thing is that large of an issue for you. There seems to be some real fundamental trust issues under there, or self-esteem issues, or something... something is clearly clouding the issue in a way that could be very detrimental to the relationship over the long-term.
Most Helpful Girl
If he hid it from you then its understandable to feel like that , but he didn't. You need to just forget about it.