Perhaps Sirius is in some kind of danger or under a curse himself. And the only way to save him or break the curse is for Harry to 'please' him in ways that are not entirely clear at first. This could be a complex plot device where Harry has to figure out what it really means to 'please' Sirius and how it relates to the overall magical situation.
One possible reason could be some sort of dark magic or a spell that has been cast on Harry Potter. Maybe an enemy of Sirius wants to use Harry to get to Sirius through this strange compulsion.
Well, one possible reason could be a dark magic spell that forces Harry to do such a thing against his will. Maybe a powerful dark wizard has put a compulsion charm on him. Another reason could be a misinterpretation of a prophecy that makes someone believe Harry must perform this act to prevent a greater evil. However, this is a very strange and dark concept for a fanfiction.
One reason could be some sort of ancient magical law or prophecy. Maybe there's a hidden clause that says for the greater good of the wizarding world, they need to be married. It could be related to some power or protection that their union might bring.
One possible reason could be a misunderstanding. Maybe some false information was spread about Harry that made Sirius believe he had done something unforgivable. For example, if there were rumors that Harry was involved with dark magic in some way, Sirius, who has a strong aversion to the dark side, might be quick to disown him.
One possible reason could be miscommunication. Maybe Harry does something that Sirius misinterprets as a betrayal or a stupid mistake, leading to Sirius lashing out.
As time goes on, Harry could start to explore what it means to 'please' Sirius. He might recall their past relationship, the things Sirius likes and dislikes, and try to use that knowledge to his advantage. He may also feel a sense of loyalty towards Sirius that, despite the odd circumstances, makes him more accepting of the situation. For example, he might remember Sirius' love for adventure and try to engage in some magical adventures with him in a way that 'pleases' him.
One reason could be Harry's impulsiveness. Harry often jumps into dangerous situations without thinking, and Sirius, being his godfather and having a protective nature, would shout at him to make him realize the risks. For instance, if Harry decides to go after a Death Eater alone.
One possible reason could be that he feels unsafe at Hogwarts. Maybe there are threats from dark wizards within the school that the teachers are unable to fully protect him from. Sirius, being his godfather, represents a safe haven.
Perhaps there's a dark force at play. A dark wizard could have put a curse on him that immobilizes him on the bed. This could be part of a larger plot to either get information from him or stop him from interfering in their evil plans.
One reason could be for shock value. Writers might want to create something that stands out and grabs the attention of readers in a very different way. Another reason could be to explore a unique power dynamic between the two characters. Maybe the writer wants to show a different side of their relationship, like Sirius being in a more authoritative position over Harry in an extreme way.
In some fanfictions, it could be due to dark magic influence. A spell or a dark force might have corrupted Sirius's mind, making him turn against Harry.