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.
Harry would most likely be horrified. He has always seen Lucius as an enemy. He might start plotting ways to escape this forced marriage. He would also be worried about what this would mean for his friends and the wizarding world in general. He may try to seek help from Dumbledore or other trusted allies.
The other Hogwarts students would have a wide range of reactions. Some might be shocked and gossip about it. Neville, for example, might be sympathetic towards Harry. Draco Malfoy could use it as an opportunity to taunt Harry, but at the same time be a bit envious of the attention this situation brings. The teachers would also have different responses. McGonagall might be worried about the impact on Harry's education and future.
At first, Harry might be extremely confused. He trusts Sirius to a certain extent, but being kidnapped is still a shock. He would probably ask Sirius a lot of questions like 'Why are you doing this?' and 'Where are we going?'
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 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.
At the start, Harry would likely be quite taken aback. He has his own identity as a boy wizard, and this new situation where he is forced to dress as a girl is very alien to him. He may feel a sense of humiliation at first, especially if there are others around to witness this. He might also be angry at the person or situation that is forcing this upon him, and his first reaction could be to try and fight against it in some way, whether it's through words or actions.
Harry may also feel a sense of adventure. Despite the situation, he's used to facing strange and dangerous things. He might start to think about how he can use this situation to his advantage, like finding out more about his parents or the secrets of the wizarding world that Sirius might know. He could be a bit excited as well as scared. For example, he might be thinking that this could be a chance to get away from the Dursleys for good.
Some of their friends might think it's a strange situation. Luna could see it as a very interesting and magical development, believing that there must be some deeper, cosmic reason for it. Seamus and Dean might be a bit skeptical at first but would warm up to the idea as they see Harry and Hermione making the best of the situation. The Weasley family as a whole would probably have a mix of reactions, from Molly's motherly concern to Fred and George's attempts to find the humor in it.
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.