Characters are words on a page. They're less than toys, they aren't even objects. They're concepts. Thoughts. You cannot actually hurt or do harm to a character because they do not exist.
Is there a lot of fucked up fiction out there that I would never read? Yes.
Does reading or writing that fiction hurt anyone? No.
Banning fiction that you personally find distasteful is the first step towards Nazi-era book burning. As the old adage goes: don't like? Don't read.
Except when a character is played by a real underage actor and that someone writing the characters is imagining those real underage bodies doing very inappropriate things.
TLDR; you start writing about an underage body doing sensual things that you enjoy, flat out, you are enjoying the images of an underage body.
That's it.
