When the movie came out, none of the Bishops decided to "ban" the movie.
Many times, over the years, movies have come out, which the Church has told its parishioners to NOT attend.
So, EVEN IF the Bishops "ban" the movie, it does NOT make it a sin to watch it.
God determines what is (and is NOT) a sin.
The same is for a "dirty" movie.
Some couples may watch it, to spice up their Love Life.
Other people may watch it for depravity (arguably, a sin).