This is a question that I've been pondering ever since my separation in June. My stbxh had an ea with his "first love" last spring.
Quick history, my stbxh and I dated for 2 years and were married for 3. I was an open book with him regarding my past, and I assumed I knew everything about his. I had never once heard about this "first love" until it was thrown in my face. If I ever thought that another woman still had his heart, I wouldn't have married him.
I'm leaving many details out for the sake of brevity, but my answer is this...
I don't want to be first best, second best, or any other ranking. I want to be the only one. (children/family excluded) We all have past loves, first loves, etc. Why marry someone if you're still in love with someone else?
So, I offer you the perspective from someone who has chosen to leave and move forward. We did not have children. I will never forgive him for the betrayal. For 5 years, I thought we were alone in our relationship, but now I see that I had been living in this ow's shadow the entire time.
Sweetblessings, my situation is eerily similar to yours, except we have been married over 20 years and have 3 kids. In the aftermath of my D-day. the true nature of his relationship with her came to light and there was even a point where he said "she has some kind of hold on me" which he did an about-face on a few months later but that comment hurt badly and I am not able to move past it. It also never came to light that they talked by phone periodically over the years. He claimed that she always called him and that they only talked of mundane things such as how are the kids, etc. But she called him at work and I never knew about it--so that's a betrayal in itself.
And finally, he finds himself talking to her at a high school reunion where he is already thinking things between he and I are over but instead of doing the right thing--the thing that he'd have to "man up" to do and tell me that he was feeling that way, he just allowed himself to be swept away by all of her gushing admiration (it was vomit-ville, the things she said to him!) and launched into that EA.
I honestly felt like throwing my wedding ring and everything he'd ever given me into the toilet (still do) because if he married me with even a small piece of his heart still clinging to someone else, then our marriage is a sham. He denies this and maintains he said those things to her, thought that way about her, because of how low he was feeling about us, but the damage is done and I will always feel like thrift shop seconds. Not the best way to try to reconstruct a marriage but it's all I've got to go on right now.