I don't really see it as a love story. At least not a classical one, or a romantic one, because clearly that's only part of it. I see it more as a drama that deals with, but doesn't entirely focus on, love. Robbie's feelings for Cecilia were what facilitated Briony's mistake/lie/act, which led to someone being punished for something they hadn't done, and the guilt she felt for the rest of her life. As a consequence, the person he loved never saw him again. But I don't think it was exclusively a love story.