As far as I can tell, people don't wake up one morning, realize they need a pet name for their inamorata(o), and set about choosing one, as if they had a baby to name. And while insight into matters of the heart can often be found in literature, surprisingly few great lovers in great novels have private names for each other. Consider Leo Tolstoy's Anna Karenina. She calls Alexei Vronsky, the man for whom she gave up everything, simply, Alexei. For his part, Vronsky lavishes endearments like "darling" and "my sweet" on his favorite horse, Frou-Frou, but rarely does he call his mistress anything other than Anna.
Mr. Knightley, in Jane Austen's Emma, is a bit more demonstrative: Once the couple are engaged, the words "Emma, my love," pass his lips. But when he asks her, "And cannot you call me 'George' now?" she replies: "Impossible! I never can call you any thing but 'Mr. Knightley.' I will not promise even to equal the elegant terseness of Mrs. Elton, by calling you Mr. K."
And we, or at least I, cheer Emma on. If she called him "Georgy-porgy," or whatever one evening, I wouldn't respect her in the morning. Well, Anna Karenina and Emma don't have any sex scenes either. Pet names, like sex, are usually best left to consenting adults to do with as they will in private.
Pet names are the tip of the iceberg of private languages, and these present couples with yet another language issue: How far to indulge this impulse? Undoubtedly, it runs deep. According to scientists, approximately 40 percent of twins under the age of five or six (and some close-in-age siblings too) have a language that they speak only with each other. An awful lot of couples I know share at least a few words of a private language, including Julian and me. Ask either one of us "Do you have a doppelganger?" and the response you'll get will seem almost as far off-topic as Raymond's response to "Why don't you pass the time by playing a little solitaire?" in the original Manchurian Candidate.