In fanfiction, such a plot might occur due to the writer wanting to create drama. They might twist the relationships in the Naruto universe to explore extreme emotional situations. For example, the writer could be trying to show how Naruto, who is usually seen as strong and reliable, would react to such a betrayal. It's a way to take the characters out of their normal characterizations and see how they adapt.
It might be that the author is exploring a different side of the characters. Temari, being a strong - willed character, could be written into a situation where she makes a wrong choice regarding her relationship with Naruto. This could be due to her own insecurities or a desire for something else that she misinterprets as more important than her relationship at that moment.
It could be that the fanfic writer is trying to combine the two universes in a rather dark way. In Fairy Tail, there are all sorts of magic and temptations. If a character from Naruto's world gets involved with someone from Fairy Tail, they might be easily cheated on. For example, if a Fairy Tail character has a magic ability to manipulate emotions, they could use it to deceive Naruto's character.
Some fans might be outraged. Naruto is a beloved character, and the idea of him being cheated on by Boruto goes against the positive father - son relationship they expect. They would see it as a distortion of the characters' true natures.
In this kind of fanfiction, Naruto might become very distrustful. He could isolate himself from his friends for a while as he tries to deal with the pain of being cheated on. Maybe he would focus all his energy on training to forget about the heartbreak.
It could be that the fanfiction author has a different view on the characters. They might not be satisfied with the original relationship development in the Naruto series. So they come up with the idea of Hinata cheating on Naruto to show an alternative scenario. For example, they could be trying to show how Naruto would deal with betrayal, which is a very different aspect from the original story where Naruto is mainly about his growth as a ninja and his positive relationships.
He might cut off the person who cheated on him completely. He's a hero with strong morals, so he may not tolerate that kind of behavior easily.
Perhaps in the fanfic, Ichigo has changed a lot over time. He might have become too focused on his power and his battles. His partner could feel left out and alone. There could be someone else in their life who offers them the attention and affection they crave. In this case, they might cheat on Ichigo. Fanfic writers often like to take characters out of their comfort zones and explore how they react to difficult situations like this.
Ino could be a very complex character in this fanfiction. It's possible that she cheats on Naruto because she has a fear of commitment. Naruto is a very passionate and all - in kind of person, and that might scare Ino. So, to avoid getting too deep into the relationship, she cheats. Or perhaps there's a power struggle at play. She might think that being with a different person would give her more influence or status in the ninja world. And there could be a hidden agenda that she's following, something that she can't achieve with Naruto but believes she can with someone else.
In this fanfiction scenario, Naruto might seek advice from his friends. Sasuke could be rather cynical about it, saying something like 'I knew she wasn't trustworthy.' While Sakura might be more empathetic and try to comfort Naruto. Naruto could also start to have flashbacks of his relationship with Anko and notice all the signs he missed that indicated she was cheating.
In a 'naruto saves boruto fanfiction', Naruto might use his powerful jutsu like Rasengan to fend off enemies threatening Boruto. He could also use his experience and wisdom to guide Boruto out of a difficult situation, perhaps one involving a powerful new villain in the ninja world.