It's not appropriate. The 'Harry Potter' series has a certain moral and ethical framework. Fanfics that involve sexual scenarios between its characters distort this framework. Moreover, Bellatrix is a villain and such a relationship in a fanfic is not only inappropriate but also a strange and unethical interpretation of the characters.
Well, power dynamics is a big theme. Bellatrix is a powerful witch, and Harry has his own unique powers. Fanfics might show how they use or struggle with their powers in relation to each other. Also, the theme of forbidden love sometimes pops up. The idea that Harry and Bellatrix, who are on opposite sides, could have some sort of romantic or at least a very strong emotional connection that goes against the norms of their world.
In a Bellatrix and child Harry Potter fanfic, the author might explore the idea of Bellatrix having a change of heart towards Harry. It could start with her seeing some of James Potter in the young Harry. This could lead to a series of events where she tries to protect Harry from other Death Eaters, which would be a huge deviation from her character in the original series. However, it would make for a very interesting and unique fanfic.
One great Bellatrix fanfic is 'The Redemption of Bellatrix'. It shows a different side of her, exploring the possibility of her having a change of heart. It's really interesting to see how the author weaves a new story around her character.
In a time - travel Bellatrix fanfic, she might go back to the time when she was first introduced to the Dark Arts. This could show a different side of her, perhaps a more innocent - looking but still dark - inclined Bellatrix. She could interact with younger versions of other Death Eaters and form different alliances that change the course of the future in the wizarding world.
Character consistency. Bellatrix is a very complex character, and in a good fanfic, she should still retain her wild and somewhat unhinged nature, while Harry should also be true to his character. For example, his sense of justice should be there.
One reason could be revenge. Bellatrix might want to get back at Harry for causing problems for Voldemort and the Death Eaters. Kissing him could be a form of humiliation. Another reason could be a power play. By kissing Harry, she shows that she can get close to him and has some sort of control over the situation.
It can be a negative influence on the 'Harry Potter' fandom. It can make younger fans uncomfortable as they might come across it accidentally. It also distorts the characters' original personalities. Harry is a hero and Bellatrix is a deranged villain, and this kind of inappropriate relationship in a fanfic goes against the essence of the original story.
Often, a key element is the power dynamic. Bellatrix is a powerful and dark witch, and Harry has his own unique power. Their interactions in these fanfics play on this power struggle and attraction. For example, in some, Harry might be drawn to Bellatrix's wild and unbridled power, while she is intrigued by his potential.
Well, in a 'Harry impregnates Bellatrix' fanfic, it might explore the chaos that would ensue in the wizarding world. Harry's friends would be horrified and confused. Bellatrix, being who she is, might have her own dark motives for allowing or even causing such a situation. It would be a very complex and disturbing story that would likely rewrite many established character traits and relationships in the 'Harry Potter' universe.