All I know is, in my experience, when the looks are grossly divergent, pay grade is involved a lot in what I have seen.
Every woman I have ever met, always says "money isn't important"... but every single one of them was told by their mother, and their mother before them, and has told their daughters if they have them....
"Its just as easy to fall in love with a rich man as a poor one."
I'm not suggesting all women are superficial, but in my personal experience, when you see a couple where looks are grossly mismatched and the wife is georgeous and the guy is much lower on the scale... $$$ is the most consistent reason... The #2 on the list behind money is women who for various reasons have lower self esteem... Now I am sure there are hundreds of other reasons, as human relationships are always complex, but in terms of the top 2 in my personal exposure and experience.. that's them.