until Sensei is no longer a shithead ruining everyone's lives and when he finally becomes a good person and learns to love - the world moves on and everyone is OK.
And how will everyone be happy if in that world Sensei - the only opportunity for a happy life?
If Sensei will love only one girl, everyone else will be unhappy.
Sensei is a good guy, he makes all the girls happy one way or another. He gives them the meaning of life. Without it, in the world of the game, they do not make sense, and their life will be empty (most likely they will simply disappear during the next reset).
By the way, people who think that Amy has problems, themselves have problems. She is almost the perfect wife and the best girl in the game, that Sensei can trust. Other girls have other priorities in life, which means that they can not be trusted (for now).