From what I understood of the story, the absolute unit of a man, Sasamoto, was dragged to a toilet in the public partition he works at by this broad he first saw in a train while going to work. There, she threatens Sasamoto to have sex or she would cry rape. After the deed is done, he gets obssessed with the lady and stalks her for more sex. During his stalking, he discovers that she has sex with at least one more man on a regular basis (possibly way more), but still wants to fuck her, fact that she plans to use for her advantage, considering this chapter.
This obssession of his has some sort of root on his view (I'm probably mistaken on this part, it's been a while since I read the earlier chapters) that everyone besides him is trash and that women are entitled to like him or something.
Some chapters imply that the woman is working with some bigger organization (considering how bare-bones her apartment is and the old man who watches Sasamoto and the woman have sex through a camera in her apartment) and she may be an assassin/killer, considering that chapter with the dead body on the park. The fact that she asks Sasamoto to delete her address information from the system further supports these assumptions.
TL;DR: Guy is obssessed with the idea of having sex with this woman who may or may not be an assassin for a bigger organization.