You are correct, the postpartum depression does not excuse it but it does offer a possible explanation. Sometimes, when dissatified with a relationship... for whatever reason, people first take it personally and look elsewhere... the grass might be greener sort of thing. Not necessarily that they would act upon it... although some people do, hence affairs, but to know they have options can be a boost to the fragile ego. Unfortunately it sometimes takes the hurt of the one they love, or fear of losing a relationship to snap them into reality and realize they need to put that energy back into the relationship instead.
Good that you discussed it some more and he was able to understand how it made you feel.
Keep the communication going... wishing you much luck!

