I'm going to have a bit of a lengthly post to you, but I really, really want you to hear me out.
First, I understand how you are feeling, COMPLETELY. I feel your pain, and I know what must be going through your mind. I lived this, probably to a greater extent, and I want to share my story and opinion before you make this decision.
My ex left me for OW when our daughter was 6 months old. I found later, that the cheating began in my pregnancy, my pregnancy that was on some levels a miracle. He left me at a time I was forming a bond with my daughter for this woman, brought her into our D's life almost immediately, and went onto marry her by the time our D was 3. I actually did not meet OW until after they were married. I didn't do it until I absolutely had to. My ex had surgery, was laid up, and I had to meet to exchange my D with her. Soon after that we had our first event together when my daughter graduated preschool.
I was about the most resistant person to OW. She was a homewrecker who stole family. I hated her with everything I had. But reality was, as long as they stayed together, she was going to be a part of my daughters life forever. And at this point, honestly, even if they divorced, and I'd never believed I would say this, I'd keep her in my daughter's life, even if he didn't.
My D7 has lots of events. she's in cheerleading and dance, and I know I have to see her at her games and recitals. Would I ever ever miss any of these events because she is there? OMG no! That would be the worst thing I could do to my daughter and myself. The wedding, the first baby..... we will all be there.
You actually have a gift in all of this. Your kids are grown. Wedding and grandkids are the only things you have to share. I desperately urge you not to miss your son's wedding even if it is held at her house. Then she wins, and you and your son lose. Lose out on one of the biggest days of your son's life. You cannot hand that power over to her. It is really not worth it in the least.
However, there is no reason for you to have lunch with the both of them. Or even talk to OM if you really don't want to. I do agree that if this is a financial reason that your son is choosing your ex's home, you can explain it makes you feel uncomfortable, explore other options with him, or offer to help financially. But if there is no other option, I really would urge you not to miss your son's wedding.
Why let them take really important special life events away from you and your son? Why give them that much power?
I say this all, because I have and am currently living it, and probably will be for the rest of my life, and I'd be dammed if those two ever take away precious moments in my my life and my daughter's life.
