What is too bawdy, too immodest, too immoral to depict in a figment of film (assuming for the moment that the state even has legitimate authority in this area)?

One of the greatest films ever made is a comedy depicting the combination of psychosis, greed, incompetence, and bigotry bringing about mass murder and nuclear holocaust, culminating with the characters planning orgies in a mineshaft.

If depicting _that_ is OK (and it is - Dr. Strangelove is one of the finest in the medium, not only in its commentary on war, but its commentary on film), how in tarnation can adult actors pretending to be step-siblings cross the line?