I don't know what to tell you here. Yes, Sandra did this without asking you beforehand, she is not your puppet even though she loves you. And yes, Sandra and MC don't spend every free minute together but they share at least every morning and every night. I can't give Sandra an event every day because I have other characters where the Story should progress ( and in fact this was the first time I heard a complain about this ). She makes breakfast every morning by the way
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The iniative thing is more on an narritive point of view. Why should MC push things if the girls can push themself onto him? I prefer it this way, that the girls will start most of the sexual stuff.
Like I said, Sandra and MC already have a set relationship and spend a lot of time together. Of course someone like Lucy or Alice will get more time alone with MC because their story is still progressing. That doesn't mean that Sandra is not getting anything ( as you've seen on the event where she asks MC if he wants children ).
I get your complaints but at this point it seems a bit nitpicky. The story with all characters will progress and laying the focus on one girl would only hurt it. They will spend time together but it wont be as much as it was in the beginning to give other characters room to shine ( reminder: Sandra still is the only character that has two full pages of NSFW events in the gallery ).