At the end of the day, anything that involves a minor's genitalia and interfering in their sex life is always going to be rather subjective (unless it is obvious the writer is getting off to it and/or expects the readers to do so). I doubt there is a clear answer across jurisdictions, and even within a jurisdiction different judges may draw different lines.
I would say that care should generally be taken to stay away from erotic writing though. If we invite minors (even 17-year-olds) to discuss sexual matters, erotic content, even if the people whom it is about are above 21, might be i bad taste.
And then there are the tons of edge cases. When is it erotic to discuss how a lack of sexual relief affects your life, and stuff like that.