You may call porn an expression but in reality its an audio/visual depiction of the human sex act. Sort of like a documentary. Would you rush to legitimize a movie made about someone defecating all over the place? How about bondage and group sex acts? Are they just an expression, too?
Sorry, the movie did make history. For a time, porn became part of our culture, openly discussed and experienced by many. Note - in 2002 dollars, $600 million is 2 and 1/2 billion dollars. Any recent movies make over $2 billion?