I liked the movie but wasn't so sold on that boyfriend character

First we (and McAdams) are shown that he might be a guy who sleeps with women and doesn't call them back, and then he's completely lovable?? What happened with that? How come that seemed to bother McAdams one day, and then it becomes a complete non-issue in the next??

It would have been better for the fans to emphasize with her if she first sorted that out instead of simply sleeping with him regardless. Kinda makes it seem as if she doesn't care if she becomes just 'one of those girls' to that guy, and that all she wants is just sex (which is fine, but doesn't really seem to fit the character that the writers were going for).