This is tagged as "cheating girlfriend" and "infidelity" on mangaupdates, but this doesn't seem like there's any cheating or infidelity because so far everything is consentual. Does she actually cheat on him in the later chapters?
Watch. There will be people whining about this. I guess having a manga that actually portrays a realistic tragic story is... LE BAD! Maybe those people who don't like the story because it's not some happy go lucky CGDCT moeslop should go back to plebbit.
What would the Japanese police do in this situation if she actually went to the cops? In America at least they would consider this a serious assault and Mao would be immediately visited by police and I think it would be the same in JP. Also I feel like the reactions from the other women are...