Ok i'll agree the majority of the girls are BI but I diagree with your every girl being BI part correct me if im wrong there are atleast 2 that are not BI that I can think of straight away
Apple and Sophia.
And if I'm not wrong then you may be right when you said " I don't feel like every girl on the planet is bi".
You're right, of the AD main harem girls,
Sara/(Sophia) and Apple are not bi. Of side girls/one-offs, there aren't any indications that
Yuizumi or Sharon are bi, either. (Edit in 2023:
Yuizumi confirmed to be straight in Ch13, and as BadMustard says immediately below,
Louise is also straight.)
That said, in my personal experience, a lot of/most of the adult women I've been friends with/been intimate with have been bi. My own wife is basically lesbian except for me and other very androgynous/feminine men. Note that all/most of the girls in the game know each other one way or another, even outside of the MC connection. LGBT communities tend to cluster together over time, so it's completely reasonable that most of the women the MC has met through Q&A and the hospital could be a similar cluster of mostly-bi women.