Well, it could be that in this fanfiction, Naruto has some sort of hidden agenda. He might call Sakura ugly as a form of reverse psychology. He could be thinking that by doing so, she'll prove him wrong and become more confident. Or perhaps there's some miscommunication or misunderstanding between them. For example, Naruto misheard someone talking about Sakura in a negative way regarding her looks and he wrongly believes it, so he says she's ugly.
He could also get angry. After all, being misjudged like that is unfair. His anger could lead him to either lash out verbally at Sakura or he could leave the village in a fit of rage, feeling that he is not accepted even by his friends. This could then lead to a series of events where Naruto has to find his own way and Sakura realizes her mistake later on.
One possible reason could be Naruto's out - of - control power. If in the fanfiction Naruto has some new and extremely powerful abilities that he can't fully manage, Sakura might see him as a monster out of fear. For example, if he accidentally causes destruction while trying to save the village.
It might also cause Naruto to lose control more easily. If he hears such a hurtful term from someone he cares about like Sakura, it could make him more vulnerable to the Nine - Tails' influence. In the long run, it could be a turning point in his character development, either making him more bitter or more determined to show everyone, including Sakura, that he's not what she says.
One plot idea could be Sai trying to understand the deep bond between Naruto, Sakura and Sasuke. He might initially struggle as he has his own unique upbringing. For example, he could be assigned a mission with the three of them and slowly learn about their history, like Naruto's determination to bring Sasuke back and Sakura's unrequited love for Sasuke in the beginning. This could lead to Sai having a new perspective on friendship and loyalty.
He might be really hurt. Naruto has always been an outcast and being called a demon again could reopen old wounds. So he could just stand there in shock and disbelief, his eyes wide with pain.
One great thing about Sai and Sakura fanfiction is the creativity. Some stories might have them in alternate universes, like a modern - day setting where they are ordinary people. In these, Sai could be an artist and Sakura a doctor, and their paths cross in a new and exciting way.
Fanfiction about Sai and Sakura can be really interesting. It allows fans to explore different relationships and scenarios that might not have been fully explored in the original series. For example, some fanfictions might focus on their friendship, how Sai, with his unique personality, interacts with Sakura, who is strong - willed and kind. Writers can create stories where they go on adventures together, or face challenges within the ninja world that test their bond.
Well, it could strain their relationship a great deal. Sakura might feel hurt and become more distant from Naruto. She could also try to prove herself to him, which might lead to some interesting character development for her.