I don't get it either, but as I've come to this forum, I have learned that its not about us nor is it about the attractiveness of the OP. Without meaning to sound boastful, I know I have have so much more to offer than my FWH's OW and I am sure you are the same.
It seems to be about something in the WS needing more validation, boosting their self esteem, thinking they can get away with it, disconnecting sex from the rest of their life or whatever. I take some comfort that he wasn't emotionally involved and in the cold light of day would never have considered doing what he did. I have won hands down over the OW who never stood a chance with him. It was meaningless, so why should I let 10 minutes or however long it took to shag her in the back of her car to ruin my life?he still loves me and is full of remorse and will do anything to help me stop hurting. She meant nothing to him and never did. I think I find that easiest manage than if it had been a long emotional attachment with more meaning.
What is important however is that the WS works hard to get to the bottom of why they did it. At some point, they did make the decision to go ahead with the ONS. It didn't just happen without their consent. They need to know the why. If they don't work on the WH, there is always the possibility that they might do it again, shod the same circumstances arise. They need to workout the why and then ensure that they avoid similar situations. And your WS needs to share it with you, so to feel safe.