It's inappropriate because it violates the family and relationship dynamics of the 'Harry Potter' world. The series is about friendship, loyalty, and appropriate romantic relationships. This idea completely goes against those values.
In the 'Harry Potter' universe, there are established generations and relationships. Hermione is of the same generation as Ron and is his love interest. Ron's father is from an older generation. Mixing these relationships in such a sexual way is against the natural order and values presented in the story. It also goes against the general moral code that the series adheres to.
Some writers might create such fanfiction for the shock value. They want to break the established strong friendship in the original series to get readers' attention.
It might be because of a misunderstanding. In some fanfics, a false rumor could spread about Harry. Ron and Hermione, believing it to be true, might decide to distance themselves from him. Say, there's a rumor that Harry is involved with dark magic in some way, and they, being the 'good guys', think they need to stay away.
The reaction would be extremely negative. The 'Harry Potter' universe has a strong sense of family and friendship. Hermione is an important part of Ron's friend group, and Ron's dad is like a parental figure in their world. A fanfiction with this inappropriate plot would not only be against the essence of the story but also against the moral and ethical values that the series promotes. Fans would likely shun any such work.
In fanfiction, Hermione might choose Harry over Ron because Harry often has a more heroic image. He is the Boy Who Lived, and his adventures are often more central to the main plot. Hermione could be attracted to his leadership qualities and the fact that he always seems to be at the heart of the action.
Their relationship could develop through shared interests. For instance, if they both have a passion for magical inventions, they might start collaborating on some small projects at home. This would lead to them spending more time together and getting to know each other better.
Well, it could be for love. In many fanfics, their relationship has been through a lot, and leaving together is a way to start anew, away from the prying eyes and pressures of Hogwarts and the wizarding community.
In some fanfictions, Harry might be angry with Ron and Hermione if he feels they are keeping secrets from him. For example, if they discover something important about Voldemort's plans but don't share it right away, Harry could get angry. He's used to relying on them and when that trust is broken, even if it's a misunderstanding, his anger can flare up.
In some fanfictions, Ron and Hermione's wedding might face some unexpected challenges. For example, a group of dark wizards could try to disrupt the wedding. But of course, with the combined power of their friends and their own skills, they would be able to fend off the attackers. Hermione might use her brilliant mind to come up with a complex spell to protect the venue, while Ron would use his courage and loyalty to stand by her side and fight. And after the chaos is over, they would finally have their long - awaited wedding kiss, surrounded by the relieved cheers of their loved ones.
In the Harry Potter fandom, writing such fanfiction is a controversial topic. While fanfiction allows for creative exploration, this particular theme might not be in line with the spirit of the original series which focused more on friendship, adventure, and the fight against evil. Many fans would find it disrespectful to the characters' established relationships in the books.
There are plenty of alternatives. You could write about their early days of courtship, focusing on the awkward yet sweet moments as they transition from friends to more than friends. Or perhaps write about how they contribute to the rebuilding of Hogwarts after the war. You can also create stories where they mentor young students together, highlighting their growth as individuals and as a couple.