One possible reason could be that Harry has different feelings towards someone else. Maybe he has developed deep emotional bonds with another male character in the fan fiction and his heart simply doesn't belong to Ginny. Another reason might be that he is still struggling with his own identity and is not ready for a traditional marriage, especially not to Ginny.
In this gay fan fiction, Harry might refuse to marry Ginny because he has his own dreams and goals that don't align with being in a relationship with her. He could be focused on exploring his new - found feelings for a male character, which are more important to him at the moment. Also, he may feel pressured by society in the fan fiction world, which makes him reluctant to enter into a marriage with Ginny.
Maybe Harry has some unresolved issues from his past, like the trauma of losing his parents and many of his friends during the fight against Voldemort. These could make him hesitant to enter into a marriage with Ginny. He might also be worried about how the wizarding world would view their marriage, especially if there are still some who oppose him or his family.
One possible reason could be that Harry has seen so much death and destruction during the war that he is afraid of forming new, deep emotional bonds. He might worry that he could lose his loved ones again, so he refuses to marry as a way to protect himself from potential future heartbreak.
One possible reason could be their fundamental differences in personality. Harry is brave and impulsive at times, while Snape is often brooding and secretive. Their past interactions, which were mostly filled with animosity and mistrust, could also be a factor. Harry might not be able to see Snape in a romantic light due to all the times Snape was cruel to him in school.
One possible reason could be a misunderstanding. Maybe Harry overheard Hermione saying something that he misinterpreted, like she was criticizing his actions but in reality she was just worried. For example, if Hermione was expressing concerns about Harry's risky adventures in a way that Harry thought was her doubting his abilities.
One possible reason could be that Harry might feel he's been used too much in the past by Dumbledore. He could be tired of being thrust into dangerous situations without having a full understanding of the whole plan. For example, in the original series, he often felt like he was just following Dumbledore's orders blindly and it led to great personal risk and the loss of some friends. So in the fanfic, he might have decided enough was enough.
One possible reason could be that she doesn't see Ron in a romantic way. In the story, perhaps she views Ron more as a brother or a close friend. Their relationship might be based on camaraderie rather than romantic love, so she refuses to marry him.
It could be Draco. In some fan fictions, they have a complex relationship that could develop into a romantic interest. Draco's character has a certain allure and his past enmity with Harry could transform into a deep, passionate love in the context of a gay fan fiction.
One possible reason could be that Harry has grown tired of being Dumbledore's pawn. He might feel that he has been constantly pushed into dangerous situations for Dumbledore's grand plans and he just wants to focus on his own life and safety for once.
One possible reason could be that Naruto is too focused on his ninja duties. He has always been dedicated to protecting the village and his friends, and marriage might seem like a distraction from that. For example, he has to constantly be on guard against new threats and train the next generation of ninjas.
One possible reason could be that he's tired of being used by the British wizarding world. He has faced so many challenges and been through a great deal of trauma, like fighting Voldemort multiple times. He might feel that it's time for others to take responsibility and that he wants to focus on his own life for a change.