@Merilirem
I feel like some of it is author choice (don't want to write porn/rape fiction), some of it is for the sanctity or purity of the reader's mind (eg implied/cartoon violence vs gore and broken/missing limbs, permanently disfigured humans), and some of it is the "disney" effect, where they want to aim it at a wider audience, so they choose not to go into explicit details of the more mature things that would be obviously going on.
I think there's a cascading effect of "if we show rape scenes here" in more accessible material, and maybe 0.1% of the audience can't separate fiction and reality, and takes it seriously into their common sense, that's still 100 people per 100,000. And if even one decides to act on those urges, you have the spawn of a tragedy. I think rape is a lot more accessible than murder to people and has longer-lasting consequences considering that the victims don't always die. But then again, there are a lot of hollywood movies that have sexual violence in them, so meh.
For just sex, it's a market distortion that people have come to accept. If you set the age of adulthood to 18, but you want to market things to underage people, you just have to leave out the prohibited stuff. Japan is already weird in that it's not seriously censored, nor heavily policed, but you still have to adhere to the common sense of "don't sell explicit materials to people who are underage but are clearly already capable of having sex."