What are the reasons a handsome man stays with an average looking woman?

This is my brother's case and I'm trying to figure out how the dynamics works in those types of relationships, where the man is handsome with options and the woman is just average. Yet she somehow manages to get him.

My brother is 6'3, white, lawyer, outgoing and we're all wealthy. He never had a hard time getting a girlfriend. He's 34 at this moment and is basically the ideal man several girls would want. However here is what I don't understand. If anyone can explain the logic in this it would be helpful.

His fiance (their wedding is sceduled to be set for Monday May 1st) is a 5'1 short Pakistani- American with glasses and braces. This isn't to praise my brother but he would be a 9 meanwhile his fiance is a 6-7. He has dated other gfs better in looks before. He definitely must know she's average. Yet he went crazy over her, enough to propose. What's the reason a handsome man with options would marry an average woman?

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He has options but at some point he did admitted to eventually wanting a family. He didn't want to date forever.
What are the reasons a handsome man stays with an average looking woman?
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