I just found this site last night doing a Google search so I haven't read any books but I will look for it. I appreciate your suggestion.
She has brought up marriage before. I am aware that she is more religious minded than me and that is an ultimate goal of hers. I am not practicing religion at this time and marriage isn't important to me. I can practice a faithful one-person relationship (I prefer it) without a marriage certificate. At times she has brought up the issue with pressure. Lately, until me catching her with this guy, she hasn't been bringing up the issue at all and hasn't now for months.
I believe you are probably right about the ignore-the-problem approach but wow it's hard. When she gets a phonecall, I want to ask who it was. When she leaves early or gets home late (we're neighbors which makes this worse), I want to ask why. When I ask if she wants to go out to eat sometime this week and she says no... but yes, I understand what you are saying. Stop calling. Stop asking. Give it some space. But it seems like that will just allow her to get closer to the other guy and pull away more in some ways. It's a no-win situation, right?