These days, most dictionaries have broadened their treatment of sexual intercourse. They acknowledge that while the term usually refers to the penetration of the penis into the vagina, it can also be used to describe other genital contact, using expressions like "genital contact," "penetration," and the like to allow for the possibility of acts such as anal sex. But even these definitions are restricted, which is appropriate; oral sex, or masturbation, wouldn't normally be considered "sexual intercourse." The problem arises when these same dictionaries then define the word fuck (and other sexual terms) in relation to "sexual intercourse," because the word fuck is itself much broader than even these broadened definitions.
Thus, you can't fuck someone in the ass with a dildo, according to the current edition of the Merriam-Webster Collegiate Dictionary, the Random House Unabridged Dictionary, the American Heritage Dictionary, and Webster's New World Dictionary. The whore in Portnoy's Complaint "who fucks the curtain with her bare twat" can't do that, according to American Heritage, Webster's New World, Random House, or Encarta. Lesbians can't fuck each other at all, according to Webster's New World and Encarta (though if they use a strap-on, Encarta becomes OK with it). Fucking a woman's breasts is only possible according to Merriam-Webster. Finger-fucking and fist-fucking are impossible according to Webster's New World, Random House, and American Heritage; Merriam allows it, but only if it's vaginal and not anal. Only the OED, whose entry for the word I edited, defines fuck to encompass sexual acts beyond "sexual intercourse." The new edition of my book The F-Word goes into even more detail about the possibilities.
These problems have been recognized among linguists for some time. A famous 1971 essay by "Munç Wang" (a pseudonym of the syntactician Avery Andrews) discussed in tongue-in-cheek but linguistically accurate detail the syntax of sexual terms. Wang explored the syntactic constraints of various words and constructions, examining whether the object of a sexual thrust has to be an orifice, if it can be artificial, if "the orifice must be vaginoid," and whether the object must be animate. Numerous sentences are offered up for analysis of their grammaticality: "Fred fucked the log through a hole that squirrels had made," "The Wizard balled the witch's body," "Jack buggered Captain Bligh in a surgically created false cunt."
Such sentences are, understandably, rare in the real world, but Wang's essay itself can be quoted, a boon to lexicographers. If I get hit by a bus tomorrow, I will die with a certain measure of satisfaction knowing that I put the sentence "Butch fucked the mannikin through the hole he drilled in its crotch" into the Oxford English Dictionary. But almost 40 years on, most dictionaries still lack this element of comprehensiveness.
Dictionaries can also be very out of date when it comes to established words whose meanings change. If you ask most younger people what a ménage à trois is, they'd probably tell you that it's a threesome, i.e., a single sexual encounter involving three people. But the OED is the only dictionary to include this sense; the others all have only the original meaning, a household made up of three people in a sexual relationship. (Indeed, the sexual sense of threesome is absent from Merriam-Webster, American Heritage, Webster's New World, Random House, and, sadly if temporarily, the OED; only Encarta includes it.)
Dictionary editors have made greater efforts in recent years to ensure that definitions and example sentences don't reflect stereotypes—to eliminate phrases such as "doctors and their wives," for example. In some cases, these efforts have improved the accuracy of sexual definitions. As we've seen, the earliest modern dictionary definition of fuck allows the active role to be performed only by a man; a woman can't fuck. Since then, dictionaries have taken a broader view.
But not all uses are this clear. Can a woman ball a man? Can she prong him? (Wearing a strap-on, presumably.) Can she prong another woman? If it's only theoretical, how hard must you search to try to find a concrete example of a female pronger? This may seem pointlessly silly—all the more so when you learn that the very earliest example of the sexual sense of prong, from a privately printed collection of limericks, features a female pronger, a noblewoman who prongs her "wenches" with her 6-inch clitoris.
However, the issues raised—how much attention must be paid to unusual uses? how should we cover stereotypes?—are very real and must be dealt with every day by dictionary editors. Whether the question is "Can a woman rail her boyfriend?" or "Should this definition refer to the quarterback as 'he or she' to be inclusive?", the decisions have real ramifications. But while it can be stressful deciding how exactly to put sex in the dictionary, it's not without rewards: It's certainly much more fun than worrying about pesky sound changes in Middle English diphthongs.