Often, Harry reacts with anger. He's not one to take being cheated on lightly, whether it's in a relationship or in a competition. His Gryffindor pride kicks in, and he'll want to confront Draco immediately.
Harry can also be quite hurt. If he had trusted Draco, especially if they were in a relationship in the fanfiction, the betrayal cuts deep. He might become withdrawn for a while, not knowing who to trust and feeling like a fool for believing in Draco.
Sometimes, Harry reacts with determination. He decides to use his wits and skills to get even or to expose Draco's cheating. He might start investigating on his own, using his knowledge of the wizarding world and his friends' help to find evidence against Draco.
Harry might initially be very angry and hurt. He could cut off all communication with Draco and become withdrawn. In some fanfics, he would even start to avoid places where he might run into Draco.
However, there are also fans who might be very opposed to it. Since it goes against the original character relationships in the books, they might feel that it's disrespectful to the canon. They prefer to stick to the more traditional pairings and storylines.
He could also react with suspicion. Draco might think that Harry has some ulterior motive, especially if the confession is of a more positive nature like friendship or love. He would question Harry's sincerity and might even test him in some way.
Harry's reaction can be complex. At first, he may show immediate resistance, arguing that he's been wrongly accused. When he realizes that arguing won't get him anywhere, he might become sullen and withdrawn. However, this sullenness is also a sign of his determination not to let them break his spirit. He'll keep thinking of ways to prove his innocence or to show that their punishment is unjust. In some fanfictions, Harry also uses these punishments as a motivation to become better at magic, so that he can one day stand up to Snape and Draco more effectively.
Ginny might initially be heartbroken. She could retreat into herself, feeling betrayed and confused. In some fanfictions, she may cut off all contact with Harry immediately.
Harry might be confused at first. He doesn't understand why Draco would think he's a doll fan. So he might just shrug it off and not pay much attention, thinking Draco is just being his usual difficult self.
Harry might be initially shocked. He's used to seeing Draco as this tough, arrogant Slytherin. But then, he could feel a bit of pity. He might approach Draco tentatively and ask what's wrong. Harry has a good heart, and despite their past differences, he wouldn't just leave Draco crying. He may even offer some words of comfort or a handkerchief if he has one.
Harry might be initially angry. After all, he's been hurt. But he could also be a bit confused, especially if he thought Draco didn't mean to do it. In some cases, he might just walk away to cool off and think about what happened.
Harry could be initially shocked and hurt. After all, having your potential mate reject you is a very painful experience. He might start to question himself and wonder what he did wrong. Then, he could become angry. Given his Gryffindor nature, he might confront Draco about the rejection and demand an explanation.
Draco might become withdrawn. He could isolate himself from his friends and spend more time alone in the Slytherin common room, brooding over what Harry has done to him.