The question is, would readers even know what tags to search for if they wanted to read something that contains sex? And would authors know how to tag it or would there pop up all kinds of different tags for the same thing such as "smut", "explicit", "hot", "mature", "sex", "erotic", "R18" etc.
It is meaningless for us to tag the novel stuff if no one searches for that tag. It would be much simpler for readers and writers alike if Tapas just implemented search filtering with a drop down to select "General Audience", "18+" or "All". And then all series with chapters that are marked mature would be included automatically in the 18+ search.