As we note in "The Book," A Farewell to Arms was first published in Scribner’s Magazine as a series of installments. When the issue containing the first installment of the novel came out, it was banned from Boston newsstands because the novel was accused of being pornographic. This was a pretty big compliment to Hemingway, since we don’t get any graphic descriptions of sex at all. Looks like the old theory of omission (see "Style" for more) was working well.
For example, in the scene where Catherine and Frederic get a motel room in Milan before he has to go back to the front, she says she feel like a whore, but it’s pretty hard to tell when they actually have sex. We think it must have been after he kisses her, but before they eat the woodcock (we kid you not, that’s what they eat). The only reason we think it happened then is because we go from them kissing and talking to them eating dinner, and at dinner Catherine is wearing Frederic’s "tunic over her shoulders." Was she naked under his tunic? Now that’s anybody’s guess.
They might have had sex again after dinner, because Frederic tells us "the waiter came and took our things. After a while we were very still and we could hear the rain." Our guess is that they had sex in the omitted sentence or sentences in between the two we quote above. Some aspects of Frederic and Catherine’s relationship are private. And, if Rinaldi is right, Catherine and Frederic’s sex life isn’t just private, but "sacred," and therefore off limits to the reader.