There isn't any sex in this novel, and only steamy parts are the love scenes between Catherine and Heathcliff, and even those tend to be a little maudlin. The reason this novel gets an R is that the suggested sexuality is so darn perverse, involving both incest and necrophilia. After all, aren't Heathcliff and Catherine brother and sister, if not by blood (though even that is possible), at least by name. English law probably would not have allowed them to be married, so there could be both emotional and legal incest going on.
On top of the suggested incest, there is certainly some form of adultery transpiring between Heathcliff and Catherine after she marries Edgar. And finally, why exactly did Heathcliff get a spade and dig up her body the day she was buried? Who knows, but the scene seems a little suggestive.