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.
Another reason might be some kind of external influence, like a powerful dark magic that clouds Harry's judgment. However, this is often a stretch from the original concept of the characters. Fanfiction writers sometimes like to explore extreme scenarios to add drama to the story.
In such a fanfiction, Ginny might cheat on Harry with Dean due to a sense of being overshadowed. Harry is often the center of attention in the wizarding world, and Ginny could feel like she's always in his shadow. Dean, on the other hand, might make her feel more special and seen as an individual. Also, it could be that Ginny and Harry have had some unresolved arguments or differences that push Ginny towards Dean.
It could also be that the writer wants to explore a different side of the characters. Maybe they think there are hidden aspects of Ginny or Harry that could lead to such a situation. For example, they might imagine Ginny having some unresolved issues that could cause her to make such a mistake. However, this is a very non - traditional view of their relationship.
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.
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.
One reason could be that the fanfiction is set in a different universe where the characters' personalities are slightly altered. So, this 'new' Harry just doesn't click with Ginny in that version. For instance, if the fanfic is a darker take on the story, Harry might be more jaded and not open to a relationship with Ginny.
In some fanfictions, Ginny may reject Harry because she wants to focus on her own personal growth. She might feel that a relationship with Harry at that time would hold her back from achieving her own goals, like becoming a great Quidditch player or an accomplished witch in her own right. So, she decides to reject him to pursue her own dreams first.
One possible reason could be that Harry is too focused on his fight against Voldemort and doesn't want to put Ginny in danger. He might think that being with her would make her a target for his enemies.
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, in the 'ginny cheats on harry fanfic', a consequence might be that Ginny loses the trust of those around her. Harry might become more guarded in future relationships. Their families would also be affected. For example, the Weasley family might have to deal with the fallout and try to understand why Ginny did such a thing. And Ginny herself could end up feeling very guilty and regretful.