What evidence suggests that Sandy does not love Sodapop as much as he loves her?

Sandy is not mentioned often in The Outsiders. We know that Soda has a girlfriend, but it's not until chapter 12 that we learn more about the relationship. Soda, the middle brother, is described by Ponyboy as "movie-star kind of handsome, the kind that people stop on the street to watch go by." Nothing much seems to bother Soda. However, towards the end, Soda witnesses another argument between Darry and Ponyboy. Soda becomes angry and frustrated and runs out of the house. On his way out, he drops an envelope. It is an unopened letter that he previously mailed to Sandy.
Darry explains to Ponyboy that Sandy is pregnant, and the baby is not Soda's. This serves as evidence that Sandy does not love Soda the way he thinks. Her rejection of his offer to marry her serves as further evidence that she doesn't feel the same about him. The return of the unopened letter and another argument between his brothers causes Soda to run out of the house.