Antoinette and Rochester's relationship is passionately physical at one point – after all, they're newly-wedded. As their relationship deteriorates, Antoinette uses sex to manipulate Rochester, and Rochester has sex with her maid to get back at her. There are also fleeting references to Antoinette's past and present affairs. There's a lot of sex in the book, but it's presented metaphorically, not explicitly – the sex takes place discreetly off the page. Thus, it's an R.