A Categorical Conundrum

I grew up in a little town in South Carolina that had a drive-in theater.  But, mind you, not a regular drive-in.  This one was special.  When my mother and my aunt drove in, my cousin and I were hidden under blankets in the back seat.  Why?  Because it showed those movies.  You know, the ones where someone moves into a new neighborhood and is greeted by the Welcome Wagon.  Before you could get back to the car with popcorn, the now naked new neighbor, the Welcome Wagon, the Postman, and the movers were grinding and grubbing all over the screen. 

(Try telling 2 pre-teen girls to sleep through that.  Also, try to explain why the forbidden children who were told to sleep were sent for the popcorn and returned with it without anyone calling the cops or Social Services.)    

The grubbing and grinding follies, if on a page instead of a movie screen, would be in the category "adults only."  I get that.  What I don't get is where the boundary begins.  When exactly does romance enter the "adults only" category?  Brotherly Love  and E-mail Enticement  both venture beyond the bedroom door.  In fact, both describe the physical encounters in graphic and - I hope - arousing, enticing and alluring detail.  Brotherly  contains a scene in a bordello with one man and several "ladies of the evening."   Neither book contains sharing of their coupling by the focal pair nor (darn it) bondage, sex toys, or overly unusual forms or foibles.  Does the writer's intent make the difference or does it take something more?  Help me out - what makes a book fit the adult only category?

By the way, I've categorized both Brotherly and E-mail  as adult only.  Does anyone have an idea whether that helps or hurts sales?  When I check out my books in the e-tailers, some (most) of the others in the same section make my stuff look and sound pretty tame.  So I got to wondering -- am I in the wrong neighborhood?

I'd appreciate someone getting out the clue gun and pointing it in my direction.