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.
Perhaps there's a dark force at work trying to manipulate the situation. They might think that by forcing Harry and Sirius together, they can disrupt the normal order of the wizarding world. It could be part of a larger scheme to gain control or cause chaos.
One reason could be political. Maybe the two clans or villages that Naruto and Sasuke are associated with want to strengthen their alliance. By marrying their most powerful members, they hope to ensure peace and cooperation.
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 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.
One reason could be a prophecy. In the wizarding world, prophecies often drive events. Maybe there's a new prophecy that says Harry must be at Hogwarts at a certain time for some great event to occur. So, the magical community, in an effort to fulfill the prophecy, forces Harry to go. It's like they are trying to shape the future according to the words of the prophecy.
One possible reason could be some sort of political or family alliance within the vampire world. The vampires might have their own complex social structures and rules. If Jasper's family or some powerful vampire faction sees an advantage in binding their families through Bella and Jasper's marriage, they could force it. Another reason might be related to some prophecy or prediction that involves Bella and Jasper together, and the vampires believe that by forcing them to marry, they can fulfill it and gain some sort of power or protection.
Maybe it's a part of a prank by some mischievous characters at Hogwarts. They might think it's funny to see Harry in a girl's outfit.
Well, he might be in shock at first. I mean, Sirius is like a godfather to him, so the idea of marrying him is completely out of the ordinary. He could be filled with disbelief and confusion, not knowing how to even begin to process this strange turn of events.
It could be someone from a rival village. Maybe as part of a peace treaty, Naruto is forced to marry a kunoichi from a village that was once at war with Konoha. This would create a lot of tension and interesting storylines as Naruto has to deal with the cultural differences and the past hostilities between the two villages.
One plotline could be that Voldemort wants to use the marriage to gain control over Harry's special powers. Another could be that it's part of a prophecy that neither of them can avoid. Or maybe it's a political move in the wizarding world to end the war between the two sides.
Maybe it's a political move within the wizarding world. The Ministry of Magic might force this marriage to unite two powerful magical forces. Harry would then have to deal with his feelings for his friends like Hermione and Ron, who would be shocked. Snape, on the other hand, would have to adjust his view of Harry from a student to a partner. They could start to find common ground in their love for Hogwarts and magic in general.