A HB I met work work and I hear about a new restaurant opening up in our town. We both express an interest in trying it out, so we set up a time and go. It ended up being just the two of us. Which I bring up because, as you see in the title, she is in a relationship (I've never met the bf). It's not a secret, they've been dating for two years and just signed a lease for an apartment.
She and I had a good time, but nothing "happened" - most of the meal was us making sense of the life and career crossroads she and I are in. Our bosses are about to offer me a deal to move across the country for work in a couple months (more money, but I also just settled in this location and like where I am), while she has a year to decide between making that same move or staying with her bf. She likes the newer location, but would probably end the relationship if she went (bf isn't moving).
But, back to the main point. If you're her bf and she goes to lunch 1-on-1 with another guy - regardless of context, what goes through your mind?