What makes smut smut?

After writing about Fanny Hill the other day, I was thinking I might do a series of blog posts about erotic books  I read, in the order that I read them.  I assumed that this post would  be about The Story of O, a BDSM book written in France in the mid-20th century.  It's a bit lighter on plot than Fanny Hill, -- and the plot that it does have is mostly sad -- but it gets points for having not an ounce of vanilla in it. 

But in between reading Fanny Hill and The Story of O, I read Judy Blume's Forever, and Jean Auel's Clan of the Cave Bear series, and a bunch of Judith Krantz's wonderful novels.  In their time they were all called smut. They certainly all have explicit sex in them.  And yet, I don't think of them as dirty books. 

What makes a book a dirty book?  It's clearly not how well-written it is.  Plenty of smut is wonderful prose, and a lot more books that have not a whisper of sex in them are terribly written.  Is it the ratio of sex to plot:?  Or is it that the purpose of the plot is to lead to the characters having sex?  Or do we just know it when we see it?