Yoongi's reaction could vary. He might be shocked initially, but then turn it into a joke. For example, he could say 'Jin, I didn't know you were into this stuff. Are you looking for inspiration?' He could also be supportive and say 'Hey, as long as it makes you happy, keep reading.'
Yoongi may have a really interesting reaction. He could start making up his own little story based on what he thinks Jin is reading. He might say 'So, Jin, is this a story about us? Are we superheroes in this fanfiction?' He would probably make Jin feel a bit more comfortable about being caught reading fanfiction by making it into a fun and creative moment. He might also ask Jin if there are any good fanfictions that he should read too.
Oh, that would be quite an interesting situation. Yoongi might be really surprised at first. He could either start teasing Jin about it, like 'Hey, Jin, what kind of crazy stories are you into now?' Or he might just laugh it off and be like 'Well, that's a new hobby, huh?'
Sirius would be deeply affected. He'd see Harry as a sort of surrogate family, and seeing Harry in such a state would make him feel like he failed in protecting him. He'd do everything he could to make Harry feel better, from getting him medical help to just sitting and talking with him for hours about what's wrong.
He could also be secretly amused. Despite his stern exterior, Snape might see a bit of his own unrequited love in their relationship. So, he might give them a stern warning but not be as severe as he usually is.
Snape might also keep what he saw to himself at first. Instead of immediately confronting them, he could choose to observe them further. He might be curious about what they were really up to and use his skills as a spy to find out more. This way, he could gather more information before deciding on the best course of action, whether it be punishment or something else entirely.
He should also try to find out where she got the stories from. If it's from the internet, he may need to set up proper parental controls. He could explain to her that there are many positive and healthy types of literature she can engage with instead, like classic novels or educational books.
Naruto's reaction would vary greatly. For stories that show his growth in new and interesting ways, he could be inspired. He might think about how he could apply some of the ideas in his real ninja training. But if there are fanfictions that are too far - fetched, he might scratch his head and wonder what the author was thinking. And if there are ones that play with his emotions in a way that's too intense, he might need some time to process it all.
He should try to be calm. First of all, he should give her some privacy immediately to let her compose herself. Then, when they are both ready to talk, he should approach the conversation with an open and non - judgmental attitude. He can say something like 'I understand this is a normal part of human sexuality, but I was a bit surprised. Can we talk about how we feel about our sexual relationship?'
She'd probably be horrified. Fanfiction can have all sorts of wild ideas and interpretations that go against her traditional views. For example, if there are relationships in the fanfiction that she doesn't approve of or characterizations that are different from what she thinks is right, she'll be very unhappy.
They might react with surprise. Fanfiction often has very different takes on their characters and stories.
If the Avengers read Naruto fanfiction, different members would have different reactions. Bruce Banner might study the way the characters control their inner power, like how Naruto controls the Nine - Tails' chakra. Steve Rogers might be more focused on the moral lessons in the stories. He would look for similarities and differences between the values in the Naruto world and their own. Natasha Romanoff could appreciate the complex relationships between the characters, just like she values her relationships within the Avengers. And Clint Barton would probably enjoy the action - packed parts, imagining how he could use some of the ninja skills in his own battles.