I think it's because he was seeing another woman whilst he was seeing carrie, because he didnt want to be tied just to carrie. In the episode carrie sees him with another woman and approaches him, he brushes it off like it's nothing and it's prefectly normal (a bit of an idiot if you ask me!!)
Hope that helps.....
SHE FINALLY RELIZED HE WAS MARRIED ND SHE STARTED DATING OTHERS (EVEN GOT ENGAGED)....ND IN THE MOVIE HE HAD GOTTEN HIS DIVORCE ND THEY WERE TOGETHER AGIAN