What I think is the character yui is jealous more than anyone else. Just bcz the protagonist didn't come to her she decided to go with stan's proposal.
Also I saw the harem mode cheat just allows u to cheat on with different women but is there an option where all the girls know and accept it. It's confusing at times who to go with or not especially with the character yui. I do one single mistake and she does her own thing.