Why did Briony want to keep Robbie and Cecilia apart?

In Ian McEwan’s Atonement, Briony Tallis is a young girl whose lie prevents her older sister Cecilia and childhood friend Robbie from ever fully pursuing their burgeoning romance.
Briony is a prolific writer, even as a child, and enjoys making up stories because this gives her a sense of control and order. Like any child, Briony has a naive understanding of the world and has perhaps-false notions of the way things are. When she reads the vulgar, sexual language in Robbie’s letter to Cecilia and then walks in on their sexual encounter in the library, Briony is appalled. She views Robbie, whom she once admired, as a sexually perverted maniac who is out to hurt Cecilia. This shows that Briony doesn’t quite comprehend the adult realities of sexual attraction.
This lack of understanding motivates her to lie about Robbie raping Lola. She could have lied about this because it seemed like a logical conclusion based on her recent revelations about Robbie. In her mind, Robbie is the most likely perpetrator because of what she thinks she knows about him. On the other hand, Briony might have lied because she thought she was protecting Cecilia, even if she knew Robbie had nothing to do with Lola’s assault. This also shows that Briony sought order and control via lying, since she thought it was the best way to set things aright in her world.
While I’m not sure Briony necessarily wanted to keep Robbie and Cecilia apart, per se, she certainly had her reasons for accomplishing just that. Of course, Briony learns that what she did was wrong and misguided, but not until after the damage has been done.