Hi A.,
I'm really sorry you're going through this. You are right in how you're feeling about this relationship with this other woman. He may not be admitting it to himself, but he obviously has some feelings for her if he's spending that much time with her. Maybe he's getting cold feet about getting married, I'm not sure what the psychological reason is behind his behavior, but you need to have a "come to Jesus" meeting with him. You need to set the boundries for him about what is appropriate friendship with another woman and what isn't appropriate.
Working with her is one thing and maybe an occasional lunch, like once or twice a month, but regular lunch visits and going out for drinks after work is completely and utterly wrong!!!! He's crossing a line and if he doesn't stop he's going to find himself in a predictament that will shatter his homelife. I've been married for 15 years. My husband occasionally goes to lunch with other woman in his office, but that is it. It's always very professional. If he were to go out after work he would bring his wife with him.
Your fiance shouldn't be drinking alone with any other woman but you!
I would sit down and explain to him what you see and how you feel and I would give him an ultimatem. I would tell him it's either you with the boys or it's her. You need to be completely honest with yourself too! He may need counseling on this to understand. Trust me on this - marriage is hard and if he's comfortable doing this before you get married then he's a risk for letting this happen when you are married. I would not marry a man who isn't totally committed to me and respects my feelings no matter how innocent he claims his friendship is.
I'm sorry to be so harsh, but I'm trying to get you to think really hard about this before you get married!!!
I would demand he stop all outside get togethers with her and I would also demand that he NOT have lunch with her ever, given this circumstance of his obvious attraction to her.
I will keep you in my prayers!