We don't really know the full extent of what's in 'the forest sex fanfic'. But if it contains sexual themes in a way that is not tasteful or appropriate for a general audience, then it's not appropriate. Fanfics are often a creative outlet, but they should also be respectful of the community and not promote vulgar or unethical content. If it focuses more on positive aspects like character development within the game's context and just has a bit of romantic or relationship exploration in a non - explicit way, then it could potentially be more acceptable.
Given that the title includes the word'sex', there's a high chance it may contain inappropriate content. In general, content that is sexually explicit is not suitable for public or wide - spread consumption, especially if it goes against the values of the original 'The Forest' game or is not in line with good taste. But again, we can't be certain until we look at it more closely.