I see both sides on this, and both sides exist in my head when I think about actually doing it. It goes something like this:
Angel on my shoulder - "You have loved your wife for as long as you've been an adult. She's been more than you could hope for in so many ways. She fucked up in a huge way, at a young age, and lied about it for a long time, but she has shown real remorse and has been a faithful wife since marriage. She is doing everything she can to improve herself and show she loves you and make it right. She doesn't deserve to deal with the pain of you sleeping with someone else. In fact, you may ruin your marriage over it, and this marriage really is as good as it gets, despite what happened so many years ago. You have a family with her and you always wanted to grow old with a happy family, enjoying holidays, making memories, living fully. Go find her right now, hug her, and think about how awful it would be to lose her. Don't fuck it up."
Devil on my shoulder (sorry Flawed if you're reading) - "Fuck that. She cheated on you and hid it from you for years. What she deserves is to be divorced. In fact, she doesn't deserve to have ever married you in the first place or have your children. She should have been kicked out of your apartment and given a one-way ticket back home, with her family wondering how she could possibly have been so awful to you. Only 6 months after moving across the country to be with her "soulmate" and she's cheating on you. She should have been on that plane back home, alone, embarrassed, ashamed, wondering "what could have been" if she didn't throw away our future for a few rides on someone else's dick.
You have been amazing to her since day one. You actually get off on making her feel as good as possible. You've never shown her anything but unconditional love. Even when propositioned for sex by a younger woman, you turned it down. You gave up any chances to experience other woman because you wanted her to do the same for you. And she didn't.
There is no justice in this outside of divorce. The closest you can get is to recoup some of what you gave up. She owes you that much. She had sex multiple times with her AP, carelessly, without protection, in ways that humiliate you, and without so much as thought about you. In fact, she had more sex with her AP than you've EVER HAD with anyone else. How fucked up is that?
What you're asking for is so much less than what she did. A one-time thing, with her knowledge and permission, without deceit, without emotions, just sex. How selfish can she possibly be, to take so much, and refuse to give so little in comparison? If this is the deal-breaker, then maybe it's worth breaking the deal over. Pack it up and find someone else, someone who can love you and be loyal and not have this kind of baggage. You deserve that."
I honestly feel like there are valid points to both sides, which is why I feel stuck. I would rather be the person who thinks like the "Angel", but I find myself keep going back to the "Devil" thoughts.
Anyway, this is just a thought experiment for now. In reality, I am not sure I could do it even if I decided I really wanted to. And if I did do it, I am sure it would not give me nearly the satisfaction that I imagine it would. I know all that. I still think about it a lot. I can't help it.
I've told my wife, her affair confession has made her a better person me a worse person. That may be the most unfair truth of it all. I want to be who I was before, and I'm just not. Knowledge of her affair has shattered a lot of my good qualities - my trust, my compassion, my desire to be "good", are all significantly diminished. It sucks to feel that way.