You can only have sex with 2 girls in the library, at the moment. The first girl and the gen store girl. You have to go there in the afternoon/evening, and call one of them to come over. Sadly, there is only 1 position with either girl. I really hope the dev adds more scenes, and makes it where you can bring most, if not, all the girls there. I'm really looking foward to this games completion, however long that may take.