The protagonist had known his friends sister for years and he still never really knew her, not until he ended up being alone with her one fateful night. As he gets to know a more personal side of her he has to do all that he can to contain the shock at what he is discovering. He learns throughout the process that he should never ever take someone at face value. Under the exterior there is so much more that you can discover about a person. He also ends up learning a lot about himself throughout the process. When it comes to sex, it makes no sense to be reserved. It makes sense to just enjoy the entire experience.