I seriously hate this kind of writing. Just because the author want to make MC feel betrayed, he use a filmsy excuse for painting his lover as a bad guy. She broke up with him because some random person spread a rumour that he's cheating, really? Was that the best reason the author can think of? Worst yet, she didnt't even try to listen to his side of the story. If they had zero trust toward each other, why were they dating in the first place? I feel like the author was to desperate to set the tone of the story and just wrote whatever came out in his mind. That's seriously lazy, IMO.
And I have lost count of how many 'girl betrayed guy' stories have been out there already. What's wrong with Japan? It's as if those authors had trauma from their real life's relationship and tried to vent it out in their fictional stories. Ironically, this kind of settings always leads to MC having his own harem and I'm pretty sure this also will be the same.