Thanks IWITW. I DID get some good points from your post.
I am definitely civil and polite to STBXW as I would be to a work associate I don't necessarily like, but have to deal with. That does not mean I act like her friend, or want to open up to her - I DON'T. But, it also doesn't mean I act like a jacka$$ - such as my STBX FIL did last night (and has MANY, MANY times before).
Funny thing is it's the first time I said something to him, and in front of everyone. Not in an angry tone, but more in a calm, yet firm and direct manner that he was simply out of line. And he was. And, I said it b/c I don't care what he, or anyone else there (except my kids) think of me. I was simply using my relatively new spine I found after the bomb.
Funny thing is, no one in the "family" ever really stands up to STBX FIL. But my kids were there, and I did not want them to think his behavior was acceptable. I would have said something even if the kids weren't there to be honest.
I don't spend time wondering why my W made her decision, and I agree with you that is a waste of time and energy.