No. Writing any form of sexual fanfiction involving children is completely inappropriate and unethical. It is a form of child exploitation and violates basic moral and legal principles.
This is a rather inappropriate and sensitive topic. Such content is often not suitable for public discussion and may not be depicted in a positive or appropriate manner in manga.
The theme of 'two girls' study and growth journey' is great. It can tell about how they help each other in academics, overcome difficulties in school, and develop their personalities. For example, they could be working on a science project together or preparing for a big exam. This kind of theme promotes positive values and can be inspiring for the readers.
Definitely not. We live in a society where there are certain standards of propriety. Sex - related stories are not suitable for public or even private sharing among girls in most cases. It can have a negative impact on the individuals involved, such as causing embarrassment or even leading to the spread of improper information. Moreover, it may not be in line with the values that society promotes, like respect, modesty and propriety.
A story about a guy and two girls having a friendly debate in the dorm room over different types of music. One likes rock, another likes pop and the guy likes classical, and they discuss the merits of each genre.
It could be a mix. Some might be based on real experiences that are then exaggerated. However, it's not ethical to spread someone's sexual story without their consent. We should respect others' privacy and not believe everything we hear. If it's a fictional story, it might be a form of erotica writing which is a genre in literature but still should be consumed in an appropriate and consensual way.
They are popular because they play on people's curiosity about the sexual lives of others, especially those who are considered 'hot' or attractive. Human nature has an element of voyeurism, and these stories seem to satisfy that to some extent.