We're pretty sure there are other elements of The God of Small Things that could qualify it for an R rating (extreme violence, for example), but sex comes up a lot. It's not always in our faces; it's often imagined or anecdotal. For instance, Mammachi gets angry when she imagines Ammu and Velutha having sex. And we learn that, when they first got together, Chacko used to make love to Margaret Kochamma with "a passion that took her breath away" (13.54).
For the most part, the sex in the novel is pretty PG-13. We don't see much of it, we just hear about it. Even when Estha and Rahel have sex as adults (which, we've got to admit, is probably worthy of an R rating in itself) it happens so quietly and with so few details that we barely notice it until it's already over.
Only in the final chapter of the book, when we learn about Ammu and Velutha's first hookup, do we get a steamy love scene. If we were watching this movie with our mom, she would probably clap her hands over our eyes at this point.