Another reason could be related to the biology aspect specifically. In the fanfiction, the author may have created a unique lore where there is something in the biology of vampires and humans that makes a bite in a biology class significant. Maybe there are certain chemical reactions that occur only in a scientific environment like a biology class that are essential for the bite to have a particular effect, such as awakening a dormant power within Bella or creating a special bond between them.
One interesting story could be where Edward biting Bella has some unforeseen biological consequences that change the nature of their relationship in unexpected ways. For example, it might give Bella a new ability related to her senses, and the fanfiction could explore how she copes with this new aspect of herself while still being in love with Edward.
You can start by looking on popular fanfiction websites like Archive of Our Own (AO3). There are often a lot of Twilight - related fanfictions there, and you can use the search function to specifically look for ones that involve Edward biting Bella and have a focus on biology. Another option is Fanfiction.net, which also has a large collection of Twilight fanfictions.
It could make other vampires in the story jealous. If Edward biting Bella is seen as a very intimate or special act, other vampires might envy the connection that Edward and Bella are forming. This could lead to some of them trying to interfere in their relationship, either by trying to separate them or by trying to get closer to Bella themselves.
Another reason might be Edward's overprotectiveness. He could have made a decision for Bella without consulting her first, like preventing her from seeing a friend. This kind of controlling behavior could make Bella feel frustrated and end up in tears.
Perhaps she has her own vision of their relationship and doesn't want it to be influenced by the often - strange and varying ideas in fanfiction. Fanfiction can be full of outlandish concepts that she doesn't want to be part of. She wants to preserve the unique and special nature of her relationship with Edward as it is in real life.
One reason could be a shift in the story's setting. If the fanfiction is set in a different time or place, Bella's values and interests might change, leading her to not like Edward. For instance, if it's a modern - day setting with different social norms, she may find Edward's old - fashioned ways off - putting.
One possible reason could be that Bella discovers a different side of Jasper that she finds more appealing. Maybe Jasper shows a deeper understanding of her in some aspects that Edward lacks. For example, Jasper's past experiences might make him more empathetic towards Bella's own struggles in a way that Edward, with his own set of vampire characteristics, doesn't quite get.
A lot of these fanfictions also deal with the idea of acceptance. Bella has to accept her new state as a vampire, and the Cullen family has to accept her as one of their own. There are often challenges and conflicts related to this acceptance process.
One possible scenario could be that Bella is initially terrified but then realizes there's a deeper connection to Edward's bite. Maybe it unlocks some sort of hidden power within her. Another could be that Edward immediately regrets his action and tries to reverse the effects. It could also be that the bite sets off a chain of events in the vampire community, as it's an unexpected and early bite.
Perhaps there are some who have a darker side to their creativity and don't fully consider the implications of writing about abuse. But this kind of fanfiction should not be encouraged as it goes against the positive nature that most fanfiction communities strive for.