Jesus fucking christ. Did anyone else notice how bad the writing was in this one? The characterization was poor, especially with the senpai. For much of the story, we never get to know who the senpai is, what her relation to the main characters is (other than the sex), or what her motives are. Rather than a fleshed out villain, she kind of just felt like a stock character. As a result, when the senpai and Ayaka meet eyes in that infamous moment, there's no real impact. The two were total strangers. It was more weird and confusing than anything.
I also thought Ayaka's reaction was completely odd. Where's the feelings of outrage, hurt, and betrayal!? Even her jealousy wasn't all that much. Kanako was her best friend. Wouldn't she be at least a little bit upset that she was keeping such a huge secret from her?
And that second part might've been even more atrocious than the first, imo. The romantic development between Kakano and the senpai was so forced. It was way too quick –all of it. They barely knew each other. And also, can we talk about how fucked up that senpai is? She comes off as a fucking rapist! "I'm so happy I could assault you"!? At that part, I was yelling, "RUN! Get the fuck outta there!" But seriously, what the hell? I definitely agree with the others who are saying the senpai was merely taking advantage of Kakano. All those "I love you"'s were lies –that includes Kakano's (there's no way you love this chick after just 5 minutes. Please.) The girl was ignorant and naive, and the senpai honed in on that. In the end, she really is still a child.
Even from a NTR fan's perspective, this story should come off weak. The writing was just no good, imo.
Maybe you just don't like NTR at all, your complain is more about how you prefer the story did, it's not about the writing.