I am re-writing the post I asked the admin previously to be erased, to better word it. (thank you admin!). Feel free to erase any part or everything of the post you believe it is not legal in your jurisdiction.
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There is a repository of sex stories with a newsgroup that has the same name as the website, for reasons unknown to me they went offline for sometime but they are back online, it is a project of the Internet Free Literature Corporation, a 501(c)(3) United States non-profit organization and they accept all kind of fictional written content. No real stories and no encouragement to do anything illegal and no pictures allowed.

I will not name the site because, although it is legal where I live, fictional child sex stories are considered child porn in other countries like in Canada and Australia, and in the US and the UK they are prosecuted under obscenity laws, this first site is hosted in the US and I don´t know how they avoid prosecution there but they operate in the open without hiding.

The link posted by "balancer" that has been rightly removed belongs to a different site but it is also managed in the open by a legal non-profit organisation registered in the USA, founded in 2007, This second site is fan-run nonprofit dedicated to the preservation of fanworks, everything is user submitted, perhaps that is how they avoid legal trouble since they only provide hosting, they don´t create any content, I don´t know, it is only a guess that the creator is the main responsible, after all the site is not dedicated to minor stories, their main purpose is preserving the history of fanworks and fan culture some of which may contain minors depending on the creator.