How would Sirius deal with the existence of Jegulus fanfiction?

Well, Sirius would have a complex reaction. On one hand, he was very close to James, and his brother was Regulus. The idea of them in a romantic relationship in fanfiction would be hard for him to wrap his head around. He might start to question how people could come up with such a thing. He might also be a bit protective of James' memory, if he thinks the fanfiction is portraying James in a way he doesn't like. However, he might also be secretly a bit intrigued, because it's such a different take on the relationships he knew, and it might make him think about James and Regulus in a new light, but he'd never admit it out loud.

He might be very angry with them at first for how they've treated Harry.

The fandom can start by having clear guidelines. For example, on fanfiction platforms, moderators can enforce rules against excessive or inappropriate abuse themes. Also, fans can encourage the creation of more positive and canon - compliant fan works to overshadow such dark content.

We should firmly oppose and condemn it. Report any such fanfiction found on relevant platforms. Platforms should also have strict rules to prevent the spread of such inappropriate content.

One way is through proper labeling. If books contain elements of abuse, they should be clearly labeled so that readers are aware of what they are getting into. This gives parents and guardians the ability to make informed decisions for younger readers.

Society should condemn it. Since it contains inappropriate sexual content, it should not be allowed to spread freely. Platforms should have strict rules to prevent its dissemination.

Society can deal with it in multiple ways. Firstly, the media and literary critics should condemn such works. By highlighting the unethical nature of 'coercion erotic novel', they can influence public opinion. Secondly, bookstores and online platforms should have strict policies against selling or promoting such content. Also, schools and educational institutions can include discussions about the ethics of literature in their curriculum to make students aware of the problem.

Sirius would probably be overjoyed at first. Harry is his godson and he cares deeply for him.

Sirius would probably be very worried and immediately try to help Harry. He might use some basic healing spells he knows.

The fan community should firmly oppose and condemn this type of content. Moderators of fan platforms should be vigilant and remove such fanfiction as soon as it is discovered.