I agree, and I also agree with this:I get why people would be upset at Koto because of what happened. I just don't agree to statements that accuse her being abusive in her relationship with Aya.
Sure, her finding replacements in the years that followed Aya's disappearance was really f'd up, but that just shows how insecure and fragile she has always been even if she tried to change her image.
We can clearly see how Koto has been holding her emotions in the past chapters, and finally the boiling point is now reached. She surely doesn't mean what she said, she just burst it out after being emotionally triggered.
What Koto said is horrible, but it's probably her emotions got the better of her. Aya has every right to run away at that moment, but after cooling their head off, they should have a talk. If Koto can see her mistake and tries to fix it, then they can go back and see if it works. Aya loves Koto after all, I'd expect her not to leave Koto over a mistake like that.If Koto truly is manipulative as you said, she wouldn't talk it out openly.
Of course, it's another story if Koto keeps pulling this shit in later chapters, then Aya is better off with anyone except the dude (Yeah I'm not reading a GL manga for het shit). But at this moment, Koto only acted on her selfish thought once, she deserves a chance to fix her mistake.