Since it's a fanfiction, the idea of Hinata cheating could be a very unorthodox and out - of - character concept. It might be the result of the author wanting to create a controversial or alternative storyline. But in the official Naruto lore, Hinata is not associated with such behavior.
Honestly, I haven't come across a 'cheating Hinata' lemon fanfic. Fanfic writers usually explore a variety of themes, but the combination of 'cheating' and a 'lemon' (which implies sexual content) for a character like Hinata seems rather unappealing and not in line with the positive aspects of the character that most fans would like to see in fan works.
Well, there might be some very niche or non - canon fanfictions out there with such a plot, but it's not a common or widely - accepted idea in the Naruto fan community. Naruto and Hinata have a well - established relationship in the main series, and Neji is Hinata's cousin, which makes such a relationship in the context of cheating even more inappropriate and not in line with the general themes of the series.
I'm not entirely sure specifically as it could be a very specific and perhaps not widely known fanfic. It might be about a character named Hinata involved in some sort of relationship drama or unfaithful situation within the context of a fictional story created by a fan. However, without actually reading it, it's hard to be more detailed.
There might be some fanfiction out there with such themes, but it's not in line with the healthy and positive representation of the Naruto characters. Sasuke and Hinata have their own established relationships in the canon, and creating such unethical scenarios in fanfiction is not a good practice.
One possible reason could be her deep love for Naruto. Despite her actions, she realizes that Naruto is the one she truly wants. Cheating goes against her values and her feelings for him, so she regrets it as she understands the harm it has caused to their relationship.
One problem is that it goes against the positive image of Hinata. Hinata is often seen as a pure and likable character, and a 'cheating' plot twists her character in a negative way. Also, the 'lemon' aspect means it may contain inappropriate sexual content which is not suitable for all audiences.
One common theme could be exploring relationship breakdowns. It might show how Naruto reacts to such a betrayal, which can be a way to explore his character in a new light. Another theme could be the consequences for Hinata herself, like losing friends or facing social ostracism within the Naruto world's community.