Okay, so there's no actual sex in Marcelo in the Real World, but we're giving it a PG-13 for all the sex talk, which gets pretty crude at times. If it were up to Marcelo, who still thinks sex is for making babies, the book would probably get a G rating and have frolicking ponies on the cover. However, the presence of Wendell and Amos steams things up considerably, and increases the amount of swear words, too.
Wendell's always talking about sex because he (supposedly) gets a lot of it, and Amos talks about it because he doesn't get any. For Wendell, it's all about the next girl he can seduce on his yacht. As for Amos, the most exciting action he's seen lately is when his neighbor's bull attempted unsuccessfully to copulate with his old-lady cow. Both of them talk like junior-high boys delighting in how offensive they can be, proving that inappropriate behavior has nothing to do with breeding (by which we mean the social-class kind, not the farm-animal kind) or age.