The example you gave of Sergey's reaction to finding out she cheats etc is exactly the point and desired outcome. There are actual consequences involved. The internal struggle of whether it's worth the risk, what could be lost if caught, going along with being blackmailed and how far to go etc are what make the cheating dynamic interesting. I guess the easiest thing would be to just not make the bf so incompetent. Obviously he will have many flaws and reasons for her to be nudged to cheat but it shouldn't be such an easy decision to the point where literally everyone wants it because he's so useless.
At this point it's hard to imagine her not carrying on and being a slut. It's stated multiple times she 'can't go back to her old self' etc so her somehow deciding to be faithful to one person now is a bit of a stretch as she goes further and further down the rabbit hole and given how much influence so many people seem to have over her. I mean the porn star and escort parts JUST started lol.
Either way just my opinion and not expecting any changes just having discussion but still a fan of the game and enjoy it and thankful to the dev!
