You make an interesting point. Though I think I know the difference between smut and art (and I have high respect for both), my art may be another's smut. My smut may be high art to some.
I could say that we could draw the line and say a 19th century nude painting doesn't have people sweating, grunting, saying "give it to me baby...harder, you $#@#$ why don't you @#%$# me in the &*%$ you $#@#$. OHHHHHHH YEAHHHHHHHHHHH."
But again, I am assuming that those things wouldn't be art to anyone.
Rats, now I'm boggled.