I've just not accepted my own actions and my own reasons. one thing for sure is that it will change me. I hope in time myself and DW can perhaps have some from of friendship again.
Sometimes I do not get MN. Ok, here goes. Your thread reminds me of another post with another lady's and I recall I cried buckets over that. Which means to me that, it is actually quite emotively strong. It is almost like baiting for me to respond. I do feel sorry for you, there is no doubt about that.
But the above, is not something that I can help you with. Because everybody has an internal gut instinct which they need to retune when a situation like this occurs. You owe it to yourself and your DS to mature from this point onwards. We cannot help you find this reason. For myself, I have never done this in my love life with the risk to family and home, but I have done the stupidest thing of having a rebound relationship as it was in my 20s. This broke me in many ways that I can never have known.
I hope for your sake that your bad action does not send your wife to go spiralling into rejection validation relationships, as I know that is your fear also. Which is what normally kicks off this stupid sexual FWB, and bad habits which so many people deny of themselves. To me, this is indeed bad karma. Most people does not realise that peace, security and joy is not easy to come by. That is how love could grow.
The fact that you posted on here, and baited heavy emotional response is almost asking for trouble. I would definitely encourage you to find the answers for the reasons so that you gain personal peace first, even before you try to help your wife. Cos a situation like this offsets both of you, and you are not likely to be the best person to help her right now either. She also need to find her peace and centre herself again, without you being there often to trigger her anxiety. Or learn to build up her own hatred, and therefore ruin her own life too by falling into a bad habit from this point onwards and be cynical.
And for the OW, she was possibly also another one of these people who also ended up in those kind of sexual politic games, and she is also a hurt woman too. That is why she put pressure on you because she is edgy. Maybe you do not see these kind of things, but take it for what it is. Sex indeed is not "love".