In some interpretations, Sherlock could be a bit hurt by it, although he would never show it outwardly. He might retreat into his mind palace for a moment to collect himself and then come back with a sharp retort. This could also lead to some character development where he starts to question how he is perceived by others and whether he should make an effort to change that perception.
He might be initially shocked. Sherlock is used to John being more understanding, so this outburst could catch him off - guard. But then he could go into his logical mode and start analyzing why John would say such a thing.
His extraordinary intelligence could be one reason. People who don't understand his quick deductions and unique way of thinking might label him a freak. For example, his ability to notice the smallest details that others miss can seem abnormal to the average person.
One possible fanfiction could be 'The Unforeseen Danger'. In this story, Sherlock and John are on a case chasing a serial killer. While investigating an abandoned building, they are ambushed. John gets stabbed while trying to protect Sherlock, and Sherlock, in his rage to save John, gets a severe blow to the head. But their strong bond helps them get through the ordeal.
Yes, there are some such fanfictions in the Sherlock fandom. However, they often involve a lot of creative and sometimes out - of - the - ordinary interpretations of the characters and their relationships.
It could be that Sherlock gets shot in a setup. Someone lures him into a trap and shoots him. His survival might then depend on his knowledge of forensics and medicine. He could use the items around him at the scene of the shooting to treat his wound temporarily while waiting for help. This event might also cause a rift in his relationship with Mycroft, who might have been involved in the case in some way without Sherlock's knowledge.
Well, a common theme is the shift in their dynamic. Since John getting pregnant is such an extraordinary event, it often leads to a reevaluation of their roles. Also, there might be a theme of secrecy and how they handle the situation in a world that doesn't quite understand it. And sometimes, it delves into the emotional journey of both Sherlock and John as they face this new and challenging situation together.
Writers often develop their relationship by showing Sherlock's initial confusion and struggle with the new situation. He might be out of his depth when it comes to dealing with Molly's pregnancy, which leads to some interesting interactions. They also might show Molly being patient with Sherlock as he tries to come to terms with becoming a father. For example, she could gently guide him through the process of doctor visits and preparing for the baby.
Maybe John was frustrated at Sherlock's eccentric behavior. Sherlock often acts in ways that are outside of normal social norms, like his extreme focus on cases and disregard for basic social niceties. This could have led John to lash out and call him a 'freak' in a moment of exasperation.