Lil, I think you pretty much hit the nail on the head. Add to that, if he does nothing and eventually *I* do something, he will be able to salve his conscious by pointing to me as the "beyotch who destroyed the marriage".
He was married briefly before we were. I don't know all the details except for his side and I'm sure she was no angel...they were both very young. But misery ensued long before she just up and left him one day. And I think he actually felt vindicated when she did that...as in "ok, SHE destroyed the marriage" (the marriage was already in deep doo-doo, but he was able to point to her and cast blame).
I suspect the same thing will happen here. It used to really bug me and that's part of what kept me here. I've moved past that a bit...but yes, I'm still in the weighing up stage. Mostly weighing up the WHEN, not the WHY.
He's said as much to me recently, BTW. "Do you know what you ar doing to 22 years of marriage when you act this way?"