Basically a similar kind of idea to what happened with Arika, and with Terena now actually thinking about it. Where after the girls have been thoroughly corrupted and trained, they show one last bit of resistance, that is ultimately shattered when they find out they're now no longer satisfied with the sex from their old cucked lover. Pretty classic NTR trope, especially for Takeda, but that's what this game is all about.
I know Arika's "official" fall event was EV 42, and Rietta's is now this one, EV 46 seemingly. But did Takeda ever say which one was meant to be Terena's? Because honestly it feels like it was all the way back in EV32 where she called Kateve her true lover. Maybe even earlier at EV26 where she declares his cock better than Kurts.