Choc- I think like everyone here that you're handling this whole situation so well, very impressed. The analogy of the gravel pile reminds me of a story from Ajahn Brahm called "a truck load of dung" link "a truck load of dung". While it doesn't pertain directly to your situation maybe you or others can find something of value in it, the analogies did remind me of each other.
I think the two questions you ask are very important questions to ask, and if I may let me take a swing at them.
1. What is she not getting from me and our marriage that makes these other guys so attractive to her??
She doesn't have to be mommy or wife to anyone she can be herself or at least not perform any role other than the one she choses for herself. Perhaps a feeling of agency apart from the family bonds, a rebellious teen type feeling of wanting to stand away from the group and define self alone. The excitement of possibilities and danger lifting her brain chemistry out of a low level depression maybe "I can change everything" and that that will make me happy. Boosting her sense of self esteem with validation from others tells her that she still has it, unfortunately validation from others is like a drug you just need more of it to get high. She gets an emotional charge a feeling from the situation that makes her feel "alive" in a way that being "good" never did.
2. And why, o why, does she push ME away, and then end up with some romantic void in her life that she has to look elsewhere to try and fill?
Sounds like a fear of intimacy and problems joining the romantic to the familiar, the erotic and the matrimonial. In the book "he's scared she's scared" the author talks about commitment phobias, and has some great insights about active and passive avoidance of intimacy. Author interview
