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.
First of all, Harry would be extremely resistant. He has his own life and his own desires, and being forced to marry Lucius is completely against his will. He may feel a sense of betrayal if the wizarding society is allowing this to happen. He'd likely go through a range of emotions from disbelief to fury. He might even consider running away to some far - off place to avoid this abhorrent situation.
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.
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 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 would probably be extremely cautious. He has a strong sense of right and wrong, and Lucius has been on the wrong side in his eyes. Harry would be analyzing Lucius' every move, trying to understand if this is some sort of new Dark Arts plot or if there's something else going on. He would also be worried about the implications for his friends and the wizarding world in general.
He might be shocked at first. Since Lucius is a complex character, he could initially see it as a weakness in Harry that he could potentially use to his advantage in the wizarding world's power games.
At first, Bella might be shocked and resistant. She could be filled with a sense of disbelief that such a thing is being forced upon her. After all, in the Twilight universe, Bella has her own feelings and desires, especially when it comes to relationships. Forced marriage goes against her free - willed nature. So, she would probably try to find a way to oppose it, perhaps by seeking help from her friends or looking for a loophole in whatever is forcing this marriage upon her.
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.
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.
Perhaps there is a political reason. Some powerful pure - blood families might be trying to strengthen their influence by uniting the Potter and Malfoy families through this marriage, even if it's by force. Another possibility is that there is a prophecy that they misinterpreted which led them to force this marriage.
Ginny could be heartbroken. She might initially be in denial, thinking that Harry will change his mind. She has loved Harry for a long time and expected a future together, so this refusal would be a huge blow to her emotions.