Can't people who are under 13 not join this site? So, like, why is there an option for PG-13. Because no one is under 13, so no one should need PG…
Bingo. It's not meant to be an indicator that you should try to be risqué in a thread; it's a reminder to keep the risquéness to a minimum.
Exactly. It's both a notice that the thread might have slightly more mature content than the rest of the forum, and a reminder to the people posting in it that it can't get X-rated.