I do my best to clearly call out content warnings in my reviews. Also, because a good deal of what I read is darker subject matter I make a point to clarify if it’s consensual or not.
Here are two examples: モテ薬 review, スマホを落としただけなのに review
As to level - the only book in the “N3/Intermediate” range I recall with adult content is Dear Friends which I admittedly both read and reviewed quite some time ago (I read it maybe a year before Natively came online?). I don’t imagine there is anything in the beginner ratings that would qualify excepting audiovisual which isn’t on the site yet.
…But I also submitted 18+ content for the initial database seed soooooo. Yes. Filters, tags, clear markins, etc would all be good.
Edit: I didn’t realize or perhaps forgot there was something that also displayed on the book page I don’t think I ever note it when adding a book, aside from one book I hesitated to submit at all and that warning wasn’t for sexual themes but for another topic entirely