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?'
On the other hand, Harry might also be a bit curious. Despite the situation, he knows Sirius well. He might wonder if there is a deeper reason behind this. So, while being cautious, he could also be listening to Sirius's explanations, trying to figure out what's really going on.
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.
Harry would probably be very defiant at first. He has faced Voldemort (Tom Riddle) before and he won't go down without a fight. He would likely try to use his magic skills to escape or at least hold Tom Riddle off until he can find a way out.
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.
Sirius might be shocked. He'd probably feel a mix of horror and protectiveness. He'd be horrified that Voldemort has used Harry in such a way and would be even more determined to keep Harry safe from Voldemort's clutches.
Well, Sirius is a very protective character. When he discovers that Harry is a Horcrux in the fanfic, he could initially be in denial. But once he accepts it, he'd likely start looking for ways to either protect Harry from Voldemort or find a way to remove the Horcrux part from Harry. He might also feel angry that Harry has been put in such a dangerous situation.
Sirius would be deeply affected. He'd see Harry as a sort of surrogate family, and seeing Harry in such a state would make him feel like he failed in protecting him. He'd do everything he could to make Harry feel better, from getting him medical help to just sitting and talking with him for hours about what's wrong.
Harry could also try to reason with Voldemort. He might remind Voldemort of the prophecies and how killing him might not be in Voldemort's best interest. He could also try to appeal to any bit of humanity that might still be left in Voldemort, although that's a long shot.
The wizarding community at large would be divided. Those who have always been skeptical of Harry, like some of the pure - blood supremacists, would use this as an excuse to smear his name further. However, Harry's true friends would be desperate to prove his innocence or, if he is actually guilty, to help him come to his senses. Ginny's friends, like Luna, would be worried sick and would do whatever they could to get her back safely.
Harry would likely be defiant. He has faced Voldemort (Tom Riddle) before and he's not one to cower easily. So, he'd probably start looking for ways to escape right away, using his wits and any small magical items he might have on him.
Harry might blush and stammer. He could say something like, 'Sirius, it's not like that.' and then quickly try to change the subject.