Why does Scarlett want to marry Ashley?
Your analysis of Ashley Wilkes is fairly dead on accurate, in my mind. The notion of Scarlett "being respectable" and living for the perceptions of others helps to compel her in seeking to marry Ashley. He is socially respectable and in this, she covets him. I would think that she is more in the idea of being "in love" as opposed to being in love with Ashley. It seems as if Scarlett is more trapped by the illusions of love than anything else. Ashley, by nature, is not excessively emotional from a commitment point of view, as he constantly vacillates between both Scarlett's passion and Melanie's loyalty. This only adds to Scarlett's illusions about Ashley. In the end, Scarlett does recognize him as sort of wimpy and abandons her illusions of loving him.
Scarlet O'Hara has been pampered all her life, and as a beautiful Southern belle, she has had the attentions of every young man within miles. That Ashley Wilkes would marry someone as plain as Melanie is a blow to Scarlett's huge ego. So she wishes to prove this fact untrue. Above all else, the character of Scarlett O'Hara does not know herself. She illustrates the line from a song of years ago, "You don't know what you've got until you lose it."
In the book Gone with the Wind, Ashley's family is well respected and stands as the prominent Southern family. Ashley is the stereotypical Southern Gentleman. Scarlet is initially attracted to him because of these features. As the story progresses Scarlet is more distraught that he loves Melanie than her. She is very pretty and quite used to getting her way. When Rhett Butler meets Scarlet he sees right through her personality and identifies that she wants what belongs to someone else. The charms of all the other men do little to amuse her. She just tolerates them because they already adore her. In some ways Scarlett may find Ashley unattractive because he isn’t exactly manly, but he still is the one man she can not have and who does not respond to her flirtations and sexuality. This makes her want him even more.