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?  

Note from Jasmine Gold: As the name of this blog indicates, I write erotica. Check out my dark, dystopian novel about naked sex slaves, Mindgames. Your darkest fantasies, with a phenomenal plot and characters you will come to think of as beloved friends. Available on Kindle and Kindle Unlimited  and in paperback.  Or read my book of short stories about hot, consensual sex, The Mature Woman's Guide to Desire, available on Kindle and Kindle Unlimited.