Thanks Indiegirl. Your reasoning makes a lot of sense.
I absolutely believe that his emotional needs are being met by various friendships (with women) right now. Especially because he spends much more time with that group of friends than with me. Often drinking, having fun and even going on group vacations.
I have no idea how to find out if this a physical affair without making this a super involved project. The only thing I know to do is start contacting his friends and asking. And that just feels like a crazy thing to do.