What with all the prostitutes in Rome, the affairs, and the groping of the nurse, we feel an "R" rating is deserved. Not only do we know that sex is taking place, but Yossarian often describes in detail these women's bodies and his lust for them.
Then there's Hungry Joe, whose name begs the question, just what is this guy hungry for? Joe goes crazy for women, wanting to capture them naked on his camera. He is animalistic in his lust.
Sex takes a violent turn with Aarfy. Smooth enough to get any woman he wants, Aarfy's seeming chivalry proves to be a charade when he rapes, kills, and tosses out the window Michaela, a pretty and innocent maid in the officers' club. Yossarian views this as one of the most heinous crimes possible and keeps insisting Aarfy will be arrested. He never is, though, which illustrates the depraved logic of Catch-22.