Script Fill: The listener is staying over at his best friend's house, has been for a week. Where lives an older sister with a reputation of being promiscuous, and all the social problems that entails.
The listener doesn't think, or at least doesn't want to believe, that the speaker is actually that way, The truth is that she kind of is, but she is quite a bit more too.
The plot gets into her reasons, which are very explicitly the reasons of a fictional woman in a fictional world and not intended to actually reflect reality.
He has sent her a romantic love letter and she ends up figuring out that he did it, (this is not revealed to the audience until later) She eventually confronts him about it. They fall in love and have very sweet vanilla sex with teases about more kinky stuff.
The general feel is of the “carnivorous” girl who is about to devour a timid, less experienced boy. Midway through, the dynamic drastically shifts to a very lovey-dovey vanilla feel.
