"I'll wait until you tell me everything."
I can't believe Riku actually stood up for himself and gave Shizuku a sane answer. She wanted to and was ready to cheat on Takuto with him, but Riku is having none of it. Either she explains herself or she breaks up with Takuto before anything with Riku can go forward. Now that her attempted seduction on him has reached a peak here and failed, I don't want to see more of the same shenanigans going forward, especially since Riku has already made his decision.
Maybe Takuto's got some dirt on Shizuku but she has nothing on him, which is why she can't just break up with him or tell Riku until some deadline comes up, likely the graduation countdown at the end of each chapter.
They don't seem to have an equal arrangement where she'd also have leverage against her "bf" which would be indicative of a pact vs blackmail. Unlike Shizuku who's always sneaking around making passes at Riku because for some reason she just can't initiate a break up, Takuto seems free to do what he wants and could even break up with her if he wanted to, but it's somehow in his best interest not to.