Oh, there's sex and nudity to be had in Stranger in a Strange Land. We've got everything from vanilla man-woman monogamy to orgies to group nudist communes. But since one of the major points of the novel is reconsidering the purpose of sex in society and as a grounds for relationships, that's to be expected.
So, why just an R? Because the sex is never too explicit. The book feels kind of like a movie that was filmed to be NC-17, but then the director and the editor had to take a hatchet to their film in editing and bring it down to an R. And not even a hard, flirting-with-the-line R.
Women walk around nude, but we are only ever told they are beautiful and are never given a detail-by-detail rundown of their more, um, private features. The closest thing we have to a full-on nude description is when people say Mike looks like the statue of a Greek god.
The sex scenes that do exist are written to leave the details on the safe side of implied, an underneath-the-covers vibe if you will. The wild and crazy stuff is mentioned and hinted at, but it's never given any on-screen time. Other than that, it's mostly people talking about sex in a very philosophical manner.
For adults and mature teenagers, certainly, but it's nothing that needs to be kept under lock and key in the curtained room of the video store, or bookstore in this case.