I'd have to agree with Serenata. While I adore the cuteness overload super happy fluffy type stories as well, I really do enjoy the "spice" of such dramatic elements. In a fictional setting, being attracted to someone you probably shouldn't be, can indeed be a very exciting element of the story/setting I think.
To me, thing about fictional works, be they romantic, erotic, or whatever, is that they are just that; fiction. They allow the reader to experience and take part in situations they can't / shouldn't / or simply don't want to experience in real life.
Personally, I feel the same way about stories like Oshikko Sensei, as I do about ultra violent stories. What is happening may often be terrible and deplorable, but to many readers, that is exciting, and they don't have to feel bad about enjoying it, because it isn't real.
I personally don't particularly enjoy the stories with younger children, like Oshikko Sensei, but I do not see any harm in them either.
TL;DR: As far as fiction goes, IMO pretty much anything goes.
Edit:
@dofudofu
The idea of criminalizing any fictional depictions of something, is essentially criminalizing certain thoughts. Laws concerning child pornography are, of course, extremely important, but because it is such a delicate subject, over-regulation is a very real issue.
last edited at May 18, 2016 8:55PM