In the 21st of his “Introductory Lectures on Psychoanalysis” — one chapter devoted to the General Theory of the Neuroses — Freud used kissing to complicate the distinction between “normal” and “perverse” sexuality, meaning, on the one hand, heterosexual genital intercourse aimed at reproduction, and on the other hand, just about everything else. The kiss was primary evidence in his argument that this separation was too simplistic: “Even a kiss can claim to be described as a perverse act,” he asserted, “since it consists in the bringing together of two oral erotogenic zones instead of two genitals. Yet no one rejects it as perverse; on the contrary, it is permitted in theatrical performances as a softened hint at the sexual act.”

Whatever the status of Freud’s insight as a theory of the human libido, there is no doubt that he was identifying, in passing, a marvelous loophole in the morals of his time, one that would only grow larger over the next century. Kissing was permissible as a hint at “the sexual act” that could not be directly represented; and in the movies, thanks to the enhancements of lighting, makeup, close-up and decoupage, it was an even broader and more suggestive hint than it was onstage. A movie kiss was also, for a long time and under various formal and informal censorship regimes, a substitute for everything else. A kiss was all the sex you could show on-screen, and it is precisely the turning of a particular, nongenital sexual activity into the whole of sexuality that fulfills Freud’s definition of perversion.

In the present, where Internet video of any imaginable sexual act is a few well-chosen search words away, we sometimes look back on old movies as artifacts of an innocent, more repressive time. But it may be more accurate to regard them as the force that made perverts of us all, by invisibly smuggling all that other stuff in through soft, innocuous hints. The prudes who wrote the Production Code that reined in Hollywood’s incipient salaciousness early in the sound era certainly suspected as much. Aware that they could not control every image and scenario, they mandated that “special care be exercised” in a number of sensitive areas, including “excessive or lustful kissing, particularly when one character or the other is a ‘heavy.' ”

But lust and excess are in the eye of the beholder, and the audience is perfectly capable of projecting onto the screen much more than what the light beam passing through nitrate will expose. A kiss is not just the chaste signifier of other, naughtier pleasures — or of socially sanctioned, baby-making marital relations. It is a gateway drug, literal proof that the scolds have always been right.

Movies have always been about sex and have always provided, under cover of harmless amusement, the tools of sexual initiation. This is an open secret. The industry, the audience and the critics conspire to pretend that something other than erotic fulfillment is the reason for the art form’s existence. And of course there are a great many ennobling and inspiring things that movies can do, other passions that can be aroused as we sit on soft chairs in the dark, surrounded by strangers. But every once in a while someone spills the popcorn.

In his poem “Ave Maria,” Frank O’Hara exhorts the “Mothers of America” to “let your kids go to the movies!” The first reason is to give Mom a chance to pursue her own adult interests: “get them out of the house so they won’t know what you’re up to.” But they will also have the chance to get up to some mischief themselves (“they may even be grateful to you/for their first sexual experience”), to cultivate “the darker joys” that blossom in the dark of the movie theater and that include the possibility of “leaving the movie before it’s over/with a pleasant stranger whose apartment is in the Heaven on Earth Bldg/near the Williamsburg Bridge.” On the other hand, if the mothers don’t listen to the poem’s advice, “the family breaks up/and your children grow old and blind in front of a TV set/seeing/movies you wouldn’t let them see when they were young.”