So much of what Chamomille Tea wrote. I went to quote some of it, but it's just all of what she wrote.
I have some things to add from WS/MH side, but let me give you a quick overview of my sitch so it will provide context with what I am going to tell you.
So, my H had an affair after mine. No permission or anything like that, but it was classic pretty much what is described in not just friends. His was 18 months, lots and lots of meetups and sex, his AP wanted my life badly. I had a two month A, emotional affair mostly with one meet up.
There is no reason to compare our affairs, I don't think his was worse than mine or any of those types of things. That stuff is completely pointless. There is no point scoring in affairs.
It's hard to ignore the idea that I brought him a crisis with my affair, he didn't cope well with it, and so he found this escape like any other WS. But, the reality is I don't know that he wouldn't have had a traumatic event or some other crisis, vulnerability or precipitating factor that would have yielded the same result down the road.
One thing about WS - even those of us who thought they would never cheat - we must not believe to our core that cheating is wrong. There is a loophole in our integrity that when push comes to shove we can bend the logic to justify it enough to do it.
That's every single WS, including my husband. Which brings me to my point. It's an error in logic to believe that cheating is so wrong that you will solve it by cheating yourself. How can he hold you accountable to a standard he does not have for himself? Right?
It's a pandora's box that is not going to cure him. There are so many things he's opening with this that you will never get the box closed again. One, there is another individual he wants to involve. What about her feelings? Do you think it's fair for him to use her to make himself feel better? And, what about feelings? She "falls in love" with him and gets hurt. He "falls in love" with her and wants to leave? He is going to be comparing how miserable he feels with you, to the light and airy fun fest with her. And, you know what? You are not completely responsible for the fact he is miserable. I do not want to walk all over his trauma, I don't mean it that way. I am just saying that my husband could have chosen a lot of things to make himself happy, instead he chose an affair and has probably been the most miserable that he has since this whole odyssey started almost 4 years ago.
So, that was my MH stuff. Here is my WS stuff.
CT hit on a lot of truths so I won't repeat all of it. She hit on a lot of things all women go through from about your age to probably your mid to late 40's. For me, it was realizing I am mine. I share this journey with H because I choose to, but this is my life and my journey. I am responsible for it. I am responsible for my own happiness. I had spent the prior two decades as a dedicated wife and mother and had given so much that I didn't know who I was or what I wanted. Mid-life is when the universe takes you by the hand and say's honey, you are running out of time - you have all these talents and unique traits. Figure out how not to waste them and how to follow your bliss. Figure yourself out, what you want.
And, as a WS the biggest thing I have had to learn to embrace is the power of self-love. Self respect. Self worth. Our longest relationship is with ourselves. And, our relationship with ourselves reflects back in all our other relationships.
I would encourage you to find that strength and love and really figure out what you want in this life.
This request is a hard pass. No. NO.
Don't be afraid of losing this man by saying no. You are likely to lose him more slowly and painfully if you agree to this anyway. You are between a rock and a hard place.
Encourage him to do IC. If he hasn't even tried that, why on earth would this be the most logical way he can heal? It sounds very much like my husband - did not do anything proactive, just got caught up in a fantasy world instead. That didn't fix shit. Not even close.
Even worse, I don't think the way your husband is talking that he would be going to therapy or evaluating his how's and whys afterwards. Working to make himself a safe partner again. How do you even begin to close that loop? What if he just enjoys it and it leads to a second affair that maybe he conducts more clandestine? What is going to stop the behavior? Affairs are highs, they feel good until they don't. What's to stop him from chasing this high again and again?
This is not the solution, and he hasn't even delved into finding some reasonable solutions. Have some self respect and don't allow the two of you to go down this path. I promise it fixes NOTHING, it makes everything infinitely worse.