I don't know, but to me she looked much more attractive in her slip, from the "save it for the liberry" scene, than she did in her bridesmaid's dress. Although Sonny could've had other women (and there's nothing to say that he didn't), maybe she was convenient. She was a friend of the familly, so they knew one another beforehand. She knew what he was, she knew that he was married, she was very undemanding, and, according to the book, she adored him. Adoration can go a long way.