Most fans would probably be outraged. Harry and Ginny's relationship is a significant and positive aspect of the Harry Potter series. Fans have grown to love them as a couple, and a fanfiction with such a negative plot would seem like a betrayal of the characters they adore.
I really hope there isn't such a fanfiction. Ginny and Harry have a strong and loving relationship in the original story, and creating a fanfiction with such a negative and unfaithful plot goes against the essence of their relationship.
One possible reason could be miscommunication. Maybe Ginny feels neglected by Harry because he's so caught up in his own heroic deeds and doesn't pay enough attention to her. So she might seek comfort or excitement elsewhere.
One possible scenario could be that Ginny meets an old acquaintance who has changed a great deal and she starts to have feelings for this person while still being with Harry. Maybe they share a common interest that she feels Harry doesn't understand. However, this goes against Ginny's strong character in the original series, so it's a very different take in fanfiction.
Well, it could be that Harry has started acting really distant from Ginny. He might be canceling their dates frequently or not being as affectionate as he used to be. In the fanfiction, Ginny might also find some strange gifts or notes that she suspects are from another girl to Harry, which makes her think he's cheating.
Well, many fans would likely be very disappointed. The original story has a certain charm and values. Ginny and Harry's relationship is based on things like love, friendship, and shared experiences. Writing them in such an improper way in fanfiction would deviate from what the fans expect and love about their relationship. It might also be seen as disrespectful to the original work by J.K. Rowling.
She might directly ask him about her suspicions. Just come out and say 'Harry, I think you're cheating on me. I've noticed you acting strange lately.'
In fanfiction, anything can happen. However, if Ginny cheats on Harry, it would likely shatter the trust in their relationship. It could lead to a downward spiral in the story, with Harry feeling betrayed and perhaps seeking revenge or just distancing himself. This could also change the dynamic among their friends and family, as everyone would be caught in the middle of this drama.
Fanfiction can be a bit wild sometimes. Regarding Ginny cheating on Dean with Harry in fanfiction, it could be that the writer wanted to create a more complex love triangle. It might also be a way to show Ginny's growing feelings for Harry in an unexpected and perhaps controversial way. But it does deviate from the more positive image of Ginny in the official books.
One possible reason could be a growing apart due to their different career paths. Harry, being deeply involved in the wizarding world's security and Ginny having her own Quidditch career. Another reason might be the stress of living in the public eye. Their relationship may not be able to withstand the constant scrutiny and pressure from the wizarding community.
It's probably not as popular as the more positive, canon - compliant fanfictions. Most fans like to see the characters in relationships that are in line with the original story, so this type of 'hate' fanfiction might have a niche audience.