I think part of it is that the IRL timeline of us getting the chapters and how long has passed in-story are pretty disparate. Something like several months? Less than a school year, certainly, IIRC.
So in that sense, that he's moved from flinching at her touching him to being okay with her legs around his shoulders in the privacy of her room, is certainly something.
200 chapters seems like a drag of a story, but narrative-wise I think it's actually not terrible. But, the sentiment of vague frustration over Seto's behavior is understandable when his reasons, even if understandable for him (like you said - high schoolers), seem silly to the reader who knows all of the context.
Maybe with the dad coming back, things will start picking up as Seto has another man even closer to Anjou to compare & contrast against, and potentially get advice from.