He most likely is simply getting some of his emotional, physical and financial needs met by you and the current family/married arrangement, while he gets whatever he feels are missing emotional (and perhaps physical) needs met by OW.
I tend to operate on three very basic, yet powerful principles:
1. "fear of loss" is the most powerful motivation to change;
2. Humans are, at our most basic, "path-of-least-resistance" creatures, and will not change if not forced to (or at least made very uncomfortable if we DON'T);
3. Affairs are highly addictive, and should be viewed as such. Until you separate the addict from the source of their addiction, they will lie to you and pretty much do anything in order to get their next "fix." They will also remain nearly entirely BLOCKED to you emotionally until they separate themselves and go thru withdrawal from their affair partner.
Put even more simply, your husband hasn't fully committed to your marriage because he hasn't needed to, and he is comfortable with the current arrangement.
