Maybe in the fanfic, Harry has some unresolved personal issues. He could be haunted by his past traumas, like the death of his parents and his battles with Voldemort, which make him feel that he's not in the right mental state to get married.
At first, their relationship might become strained. Harry, being forced into the marriage, could be cold and distant towards Ginny. However, as time passes, they might start to understand each other's situations better. For example, they could share their fears and frustrations about the forced union, which could gradually bring them closer.
The relationship might also develop in a more magical way. There could be some sort of magical bond that was created when they were forced to marry. This bond makes them more in tune with each other's emotions. As they learn to understand and control this bond, their relationship blossoms. They start to have feelings for each other that are deeper than just friendship, and eventually, they might truly fall in love.
For Harry, he may face a lot of criticism from his friends and the wizarding community. They might see him as a heartbreaker or someone who doesn't value his relationships. This could lead to a strain on his friendships, especially with the Weasley family who he is very close to.
In such a fanfic, perhaps Ginny uses some kind of magical coercion. Maybe she has found a powerful spell that bends Harry's will just enough to make him agree to the marriage. It could also be that she manipulates a situation where Harry feels he has no other choice but to marry her, like threatening to expose a secret of his that could harm his reputation.
He might think fanfic distorts the original concept.
Harry would probably become more reserved and distant from Ginny at first. As the story progresses, he might try to find a way to break free from the forced marriage situation. Ginny would have to grow as a character too. She may realize the error of her ways and try to make amends. Their friends, like Ron and Hermione, would play important roles in either supporting or opposing the forced marriage, which would also shape the development of Harry and Ginny.
One possible plotline could be Harry initially resisting the marriage due to feeling forced into it, but as he spends more time with Ginny, he discovers new sides of her and gradually falls in love. For example, they might have to work together to face some new dark magic threat in the wizarding world, and through that shared experience, their relationship deepens.
Ginny could be heartbroken. She might initially be in denial, thinking that Harry will change his mind. She has loved Harry for a long time and expected a future together, so this refusal would be a huge blow to her emotions.
Ginny could be heartbroken. She might have expected to marry Harry after all they've been through together. She could initially go into a state of shock and disbelief, not understanding why Harry has refused.
Maybe he has his own reasons. He might be tired of the responsibilities that come with being a hero. In the fanfic, perhaps he has seen the dark side of heroism, like the constant danger and the loss of personal freedom. He could also be a complex character who doesn't conform to the typical heroic image.