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.
Sherlock could simply ignore John's comment and continue with whatever he was doing, like focusing on a case. He might think that John is just being emotional and not worth his time to respond to. After all, Sherlock is often more concerned with facts and deductions than with John's temperamental outbursts.
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.
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.
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.
John might be a bit defensive at first. He's always been a bit more cautious about their relationship compared to Sherlock. He could try to downplay it in front of the Yarders, saying it's not as serious as it seems.
Sherlock would be extremely worried. He might go into a state of hyper - focus, using all his skills to find John. He would analyze every little detail, from the place where John was kidnapped to any possible witnesses.
In many Sherlock fan fictions, Sherlock branding John could be a sign of his trust or a way to get a reaction out of him. It all depends on the author's imagination and the context they create.
Sherlock might initially seem unperturbed, which could annoy Moriarty. He'd use his sharp mind to analyze Moriarty's motives and actions. He could also try to engage Moriarty in conversation to gain more information. He might say things that would make Moriarty second - guess his own plans. For example, he could point out flaws in Moriarty's previous schemes, making Moriarty angry or distracted. This would give Sherlock an opportunity to either escape or plan his next move.
It could be quite intense and maybe a bold exploration of their hidden desires. But it all depends on the author's imagination and style.
Often, they explore different relationship dynamics. With a female John, the emotional exchanges can be more nuanced. For example, the way she might communicate her thoughts and feelings to Sherlock could be different from a male John.
The fanfic could also focus on the challenges they face in society as a non - traditional family. John and Sherlock might face discrimination or skepticism from others when they try to adopt Harry. However, they persevere because of their love for Harry and each other. This could show how their strong bond helps them overcome external difficulties and build a loving home for Harry.