He could initially be in disbelief. However, as the reality sinks in, Sasuke would start to show his softer side. He might be more attentive to Sakura's needs, like making sure she eats well and rests. He would also start thinking about how to train their child to be a strong ninja in the future, while also wanting to give the child a more peaceful life than he had.
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.
Sasuke would likely be initially shocked. He's a man who has been through a lot and is used to a life of solitude and danger. But then, a sense of protectiveness would kick in. He'd become more vigilant about the safety of Sakura and their unborn child. He might also start to think about how to be a better father than his own was, in some ways, trying to break the cycle of his family's past.
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.
Sasuke would likely be a bit stunned at first. He's a complex character. But then he would become very protective. He has lost so much in his life, and he wouldn't want to lose Sakura or the baby. He might become more vigilant and even cut back on some of his more dangerous missions.
A third group of fans might simply ignore it. They are so set on the original story and the positive portrayal of Sasuke and Sakura's relationship that they don't want to engage with any fanfic that goes against that. They might consider it a disrespect to the original creators' intentions for the characters.
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.
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.
The other members of their team or the village might have mixed reactions. Some might side with Sakura, understanding her point of view, while others might think she's being hasty or that they should try to preserve the relationship for the sake of the village's stability or their own friendship circle.
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.