Scum, eh? Well it's not like he was the one that asked you to become a call girl, he's just exercising his right as a...client, yes?
Also it's Hentai that decide to have a story about an engaged woman working as a call girl, so...the cheating tag is like blackmail fodder, except is it blackmail if you're just expecting the service that you asked for?
Paying off a student loan debt, huh. Well, what can you do with the salary of a school teacher, huh? I feel a bit bad for her fiance though. I mean, sure, maybe he has some debt too, but talking things out might help, and also I doubt he'd appreciate her being in this kind of work.
Also it's Hentai, so doesn't really matter I guess. Engaged -> prostitution -> client is colleague -> blackmail -> sex -> addicted. End. A simple enough story but the background intrigues me somewhat. (Also, too bad about the protag being a jerk, he seemed somewhat sympathetic halfway through then suddenly...that moment his face turned black...)