High Position gets a high position on the list of the mangas with the most realistic characters. How many times have we seen a protag deny an invite from a woman because they didn't want to "cheat" on a girl that they are not even with? Kira Takashi seems to have a knack for making characters that are both compelling and containing characteristics that we could actually see in real life people.
It's not like Mitsuhiko Amano is doing anything wrong here. He isn't beholden to either of those girls at this point in time. If there was anything I would virtue signal about it would be that he wasn't calling out his sister's boyfriend for cheating so brazenly. But he is a horny born-again teenager so it figures he would be focused on the babes.