First off, whatever you're feeling is valid and not at all dumb. That's how feelings work, you don't control their appearance they just happen; a feeling can't be invalid because it demonstrably just happened.
That said, you can do dumb things based on your valid feelings or believe dumb things based on your valid feelings and that's what we want to steer away from.
I think an affair can be a lot about the WS wanting their ego stroked and if that's the goal you "affair down" for maximum ego kibbles. I was just talking about this in my own thread here on SI. Let me grab that.
I've been thinking about ego a lot lately. The other day on Reddit there was a thread about waywards insulting their AP during R and in the comments someone brought up the term "affair down," which is when the AP is decidedly less attractive than the BS.
My WW had previously said something to the effect of "it's not that he was bigger or better looking than you, if anything it's the opposite, but he was nice to me and paid attention to me."
The first time I heard that I heard a reassurance for me, the word "but," and then a justification for her shit choice. But in the context of the affair down idea, the whole thing is justification. She picked a guy that was more than a decade older than us, who was less attractive than me, who made less money than me (or her for that matter). A guy for whom my WW was probably the hottest chick he'd ever managed to land. It's no wonder that he fawned over her; if you get a goose that lays golden eggs you keep that bird happy as fuck. What an ego trip that must be.
There are a couple ways of thinking about this, I think.
1. Your WS didn't pick his AP except that he chose her from small selection of women (perhaps a vanishingly small selection) who were messed up enough in their own right to settle for being the OW and craved your WS attention enough that she would treat him to all the ego kibbles he so craved.
2. Your WS picked her because he knew she was messed up and what you consider the EA could also be described as grooming.
I can't tell you which of those two are true or if either of them are. If you think that it might be #2 you should talk about it in IC or MC (you're doing at least one of those right?).