One possible reason could be Dumbledore's manipulation. Dumbledore might see it as a way to keep Harry safe under Severus' watchful eye, knowing Severus' skills in magic and his loyalty (in his own complex way). Another reason could be a magical binding or an old wizarding law that, due to certain circumstances, forces Severus into this situation. For example, if Harry's magical protection is somehow linked to Severus in a way that adoption is the only solution to maintain that protection.
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.
At first, Severus would likely be extremely resistant. His cold and often bitter personality would be on full display. He might be very strict with Harry, not out of malice but because he doesn't know how else to handle the situation. However, over time, he would gradually soften. His protective instincts, which are there deep down, would start to surface. For example, he might start to show concern for Harry's well - being in small ways, like making sure he has proper potions for any ailments.
It could also be because of relationship incompatibility. Hermione is highly intelligent and driven, and Severus is a complex and often brooding character. Their different personalities might clash in the fanfiction. For instance, Hermione might feel that Severus doesn't support her academic or social pursuits enough, or Severus might find Hermione too outspoken and interfering in his solitary world.
One possible reason could be Severus' long - standing grudge against James Potter, Harry's father. Since Severus had a very difficult relationship with James, his hatred might be so deep that it clouds his judgment when it comes to Harry, who is a reminder of James. Another reason could be Severus' loyalty to Voldemort in the past. He might fear that accepting Harry as his son could put him in a difficult position with the Death Eaters or those who still hold onto the old Dark Arts ideology.
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.
It's unacceptable because rape is a form of violence and a violation of a person's most basic rights. In the context of fanfiction, it's inappropriate to use beloved characters in such a disturbing scenario.
Redemption is a common theme. Severus often has a chance to redeem himself or be redeemed in the eyes of the marauders.
One reason could be that he realizes Harry has a lot of potential and wants to nurture it. He sees that Harry is in a difficult position, being the target of Voldemort, and decides to help him despite their past differences.
It could be that Harry, through his kind heart, comes to understand Severus's complex past and motives. He realizes that Severus is not all bad and that there is a deeper connection between them. This new - found understanding and emotional bond drives Harry to save Severus from harm.
One possible plot could be that there is a powerful dark wizard who forces Harry into marriage as part of a grand scheme to gain control over the wizarding world. Maybe this wizard has some ancient magic that compels Harry and he has to find a way to break free while also dealing with the political and social ramifications of this forced union within the wizarding community.