For what it's worth, I realized after release that once the player embarks on the casual path, he is not given an opportunity to change course recover from that poor choice.
One pitfall of development is that you understand the pathing so well, it's easy to fall into the trap of assuming everyone else does too.
While I think both girls' displeasure was adequately telegraphed, it does seem unfair to punish the player in Chapter 9 for a single choice made in Chapter 6.
The result will be the v1.01 patch, which will add an additional scene. This scene will provide the MC an option to be proactive about making things right before decisions are made for him.
While it will not add new endings, or provide a casual route for both LI's, it will be far more fair for player agency.
This will not protect the MC from the consequences of cheating (while in a committed relationship) however.
I hope this helps.
I expect this patch version to be ready sometime tomorrow.