Sex in this book is mostly reserved for Robert Jordan and Maria. Maria crawls into Robert Jordan's sleeping bag the night she's met him, having exchanged few words and lots of awkward glances. After that, they can't get enough of each other: next day they have sex in the heathers that makes the earth move, followed by another romp that night. The last night Maria's not feeling up to it, so they talk instead. In the early morning, Maria wakes up feeling as good as ever, and they get one last go before being parted forever the next day.
As for the sex other people are having, we also know Pablo and Pilar have sex at least once, on the first night (Pilar smilingly says so). Beyond that, no sex happens in the three days the book recounts, but we hear various mentions of sex in flashbacks, whether it be Pilar's affair with a bullfighter, or Robert Jordan's past experiences with prostitutes, or Robert Jordan's dreams about Gretta Garbo.
Finally, people in this book are just rather open about sex. See for yourself how many sex jokes Pilar makes about Robert Jordan and Maria, or how many times one of the other guys asks how Maria is in bed.
Thing is, sex, like violence, gets Hemingway's indirect approach: he never actually describes the act itself. The first night, we just get some dialogue which makes it clear that Maria has taken her clothes off, and the scene concludes with Maria saying "Yes. Yes. Yes." In the later Robert and Maria scenes, Hemingway adopts a different approach: Robert Jordan repeating the same words repeatedly. Maybe you'd be fine with a PG-13 rating, but we're definitely leaning toward R.