I think Serena choose Charlton coz Geoffrey is not a loyal man he know tha Serena is his fiance then why did he had to have relationsip with Emily. If he want Emily the he choose Emily after that he doesn't want her then he brkup with her. He is just a confused man where he don't know who he want .Even if he want Serena and I don't think Serena will have blissfull married life.