This whole flashback arc has been bothering me. It doesn't seem to fit.
Why didn't it come up earlier than it did? It clearly had an impact on her, for meeting Senpai again to knock her into flashback mode for so many chapters. Yet in all her original contemplations of romance, sexuality, and her feelings for her mom, the things that really made the manga, it never came up.
If her innocence had been taken advantage of, why did she never think a single word about it after she took advantage of her drunk, sleeping mother to kiss her? She was agonizing in her room over it, and yet it never turned in the direction of "am I as bad?". If this encounter is as negative a memory to her as it seems, then it would be strange not for the comparison to happen when she was thinking so negatively.
In chapter 6 she even said she'd "never given much thought" to whether she liked girls, and indicated it was specifically her mom she was interested in. How does that mesh with what we're seeing here? How can she come away from what's already happened, never mind what we know happens next chapter, and not be asking herself that kind of question? She clearly has some attraction to Senpai, and I don't see how she could come out of what's coming next without being left with a BOATLOAD of introspection about that. This is the kind of thing that can reshape a person's entire view of sexuality.
It was mentioned beforehand in Ch. 9, yes, but that's still 15 chapters in, and only shortly before the arc kicked off. It doesn't feel like something the author had planned, or at least adequately foreshadowed, and yet it's turned into a central part of the series.