There would be a portion of fans who are very against it. They are so used to the idea of Sakura and Sasuke being a potential couple that any deviation from that would seem wrong to them. They might feel that it goes against the 'canon' in a way, even though it's a fanfic.
On the other hand, there would be fans who would welcome it. Maybe they never liked the pairing in the first place or they think it adds an interesting twist to the story. They could start exploring new relationships for both Sasuke and Sakura in their own fan - created stories.
A Sakura cheating on Sasuke fanfic would have a significant impact. Firstly, it would change the perception of Sakura's character for many fans. Those who view her as a loyal and loving partner would be disappointed. Secondly, it could inspire new fan art and discussions. Some fans might create art based on the angst and drama of the situation. And thirdly, it might even lead to some fans re - evaluating their love for the Naruto series as a whole if they feel the fanfic goes too far against the essence of the original relationships.
The village would probably be in shock at first. Naruto is a very important figure in the village, and his marriage is a big deal. Some might be happy that he has two strong women by his side, thinking it would strengthen the village even more. But others, especially the more traditional ones, might be against it as polygamy is not a common practice in the Naruto world. They would likely gossip and there could be a lot of hushed conversations in the streets and in the tea houses.
Sasuke could react very angrily. He might lash out at Sakura verbally, asking her how she could do such a thing. He has always been a bit of a hot - headed character, and this situation would likely trigger his temper.
Another reaction could be that Sakura tries to follow Sasuke. She believes that she can change his mind or that she can be of help to him. She might leave her own responsibilities behind for a while to chase after him, thinking that their bond is strong enough to overcome whatever is making Sasuke leave.
Sakura would probably be initially very angry. She has strong feelings for Sasuke, but kidnapping is a very extreme act. So she would likely fight back, using her medical ninjutsu skills and combat abilities to try and escape.
She might be shocked at first. Given Sasuke's past coldness towards her, it would be unexpected. But then she would be overjoyed. After all, she has loved Sasuke for a long time. She would probably start crying tears of happiness and immediately accept his confession.
On the other hand, Sasuke could be very stoic about it on the surface but internally be in turmoil. He might start to analyze his own actions and whether he had missed any signs from Sakura. He could also be the type to immediately start thinking of ways to win Sakura back if he realizes he can't stand the thought of her with someone else.
Sakura might be heartbroken at first. She has always been in love with Sasuke and to find out he's cheating would shatter her world. She could become withdrawn and focus more on her medical ninjutsu as a way to cope.
Naruto would be devastated. Sasuke is his closest friend and rival. He would likely go into a state of deep mourning at first. He might isolate himself for a while, just trying to come to terms with the loss.