Bob's marriage to Alice was unhappy; cheating with Betty was only a symptom (not a cause) of their fighting and eventual divorce. His relationship with Betty brings him happiness, but he isn't marrying her solely for her beauty.
Gender Inverted: Alice was married to Bob, but when her career took off, she had an affair with Charles, and eventually left Bob for him. Charles is younger and more handsome than Bob, and probably much younger than Alice.
Conversed: "And what happens when Betty gets too old?"
Deconstructed: Bob let his success go to his head, and that's why he ended up being unfaithful to (and eventually divorced by) Alice. He may have been unhappy when he was with her, and that led to fighting...and his negative feelings made him vulnerable to Betty's charms. Also, Betty may be young and beautiful now, but eventually she will become older, pudgier, and wrinklier...what then? And it's possible Betty only married Bob for his money...and what if she's a Black Widow?
Bob decides that Alice has been there for him through a lot while he was getting his career together, so he focuses on building a stronger relationship with her, and breaking off his illicit relationship with Betty (or turning her down in the first place.)