Question: At the end of the movie, Charles and Carrie decide to spend the rest of their lives together without being married. If Charles loves Carrie so much and wants to spend the rest of his life with her, why doesn't he just marry her?
Answer: I don't recall if it was determined whether or not they eventually married. The end showed them together with their baby and implied that they had married by then.
He specifically asked her not to be his wife.
Question: If Carrie loves Hamish so much, why does she still hang out and sleep with Charles?
Answer: Carrie was attracted to Charles and gave in to her passion; she even said she thought they had a missed opportunity. Most likely, she never truly loved Hamish, was infatuated with his status and wealth, and probably felt it was too late to back out. Carrie really didn't know the type of person he was.
Answer: Carrie had just gone through a divorce, and Charles had left a woman at the altar. They were both kind of soured on the idea of marriage.
Brian Katcher