One possible reason could be a misunderstanding. Maybe Naruto was misidentified as a threat in a tense situation. For example, if there was some sort of dark jutsu at play that clouded Kurenai's judgment and made Naruto seem like an enemy. Another reason might be that Kurenai was under some sort of mind - control. There are many villains in the Naruto universe who are capable of such things. If she was being controlled, she might unknowingly attack Naruto.
One possible storyline could be that in a mission gone wrong, Kurenai accidentally uses a jutsu that ends up hurting Naruto. Maybe she misjudges the situation and her genjutsu affects Naruto in an unexpected and harmful way.
Sakura would probably be very angry at Kurenai. She's protective of Naruto and would demand an explanation. She might even try to heal Naruto right away if it was a physical injury.
He might be shocked at first. Naruto is used to seeing Kurenai as an ally, so being hurt by her would be unexpected. He could immediately ask her why she did it. If it was a serious injury, he might struggle to stay conscious while trying to understand the situation.
Tenten could be one of the characters. She could be Naruto's friend and also have a bit of a crush on him. Ino might also be present, using her mind - related jutsu to try and understand the complex relationships in the harem. Rock Lee could show up too, providing some comical relief and also being Naruto's loyal friend in the midst of all the relationship chaos.
I'm not entirely sure if there is a specific 'Naruto cheated on by Kurenai' fanfiction. Fanfiction websites are full of a huge variety of stories, and it could be quite difficult to find one with such a specific plot. You could try searching on popular fanfiction platforms like FanFiction.net or Archive of Our Own using relevant keywords, but keep in mind that the Naruto fandom has a wide range of story themes, and this might be a rather unusual one.
One aspect to consider in this fanfiction scenario is how their marriage would affect the other characters. The young ninja in the village might look up to them as a couple. Naruto's friends like Sakura and Sasuke would have different reactions. Sakura might be happy for Naruto but also a bit envious, while Sasuke might initially be skeptical but come to respect their relationship over time. And their family life could involve Naruto teaching their future children his unique ninja skills, while Kurenai imparts her wisdom about the more subtle aspects of being a ninja.
I'm not entirely sure if there is a specific 'naruto is kurenai son fanfiction' out there. Fanfiction platforms are filled with all sorts of creative ideas, but it can be difficult to know for sure if there is one specifically for this concept. You could try searching on popular fanfiction websites like FanFiction.net or Archive of Our Own. Maybe start with searching for Naruto - Kurenai family relationship or something similar to see if any relevant stories pop up.
In most Naruto fanfictions, there is no such plot where Kurenai cheats on Naruto because they don't have that kind of relationship in the original story. Kurenai has her own story arc mainly related to Asuma. So it's very unlikely to find such a scenario in regular Naruto fanfictions.
Sakura could be one. She has always had a bit of a complex relationship with Naruto. In this harem fanfiction, she might see Naruto in a new light and become part of the harem. Another could be Hinata. Given her long - standing crush on Naruto, it would make sense for her to be in the story. Also, Tenten. She's a strong and independent ninja, and her interaction with Naruto could be an interesting addition to the harem scenario.