I don't get everyone's confusion.
Tsubasa meets Misaki and they start doing the nasty regularly. Rumors start at school about Tsubasa being a prostitute. When she comes to school with a black eye Misaki can do nothing but wonder if the rumors are true so she asks Tsubasa. Tsubasa tells Misaki the rumors are true and leaves, she then goes to Jun's house. This is where a flashback happens.
Tsubasa lives in an abusive household. One day she meets Jun. Jun provides an avenue for Tsubasa to defy her father, they pretend to be prostitutes to rob men and then start having sex with each other. Turns out Jun is a real prostitute and seems to be grooming Tsubasa to pimp her out. Jun gets two customers to have their way with Tsubasa but changes her mind when she sees how much Tsubasa doesn't want it.