[Dimestore shrink alert]: Could it be the whole abandonment thing? I know that AH and I are drawn together like magnets because we both lost our alcoholic dads at the same time in our development.
So, while we have been desperate to stay together, in the past any threat that the marriage may not work sends AH into "survival mode" relationship-wise.
Case in point: When he relapsed, he tried as hard as he could to get a few women in the wings, so that if/when I kicked him out, he wasn't alone.
Perhaps that's what your ex is doing? Just shoring up the emotional dam?
This is his deal... maybe he has a fantasy of a plug-and-play relationship, but I agree that you might want to tread lightly as far as your own kids are concerned.