Ponyboy spends most of the book deep in his own problems, from the Socs to Bob's murder to the fire at the church. Because of this, he takes his brother Soda for granted. Soda acts as the glue between his two other brothers, and he seems to take life in stride and is a source of encouragement for his brothers and friends. As it turns out, Soda is quietly dealing with his own problems—namely Sandy.
When Pony comes home from Johnny's loss, he and Darry break out into their typical fight. Since Soda is hurting, he leaves the two of them, which he typically doesn't do. Darry then fills his little brother in on Soda's situation with Sandy.
Sandy has gotten pregnant—and not by Soda. Because he loves her so much, he offers to marry her even though he is not the father. Her parents don't approve of this idea, though, and Sandy is sent to Florida for the foreseeable future.
Teenage pregnancies were certainly much more stigmatized in this time period, and Sandy doesn't have options. She leaves Soda during the same week that Pony also heads into hiding following the murder, so Soda faces doubly difficult losses in addition to the knowledge that his great love hasn't been faithful to him.