MOSCOW - What finally tipped the balance? Was it the Soviet version of Dr. Ruth describing on national television the pleasures and risks of various sexual positions? Was it the live sex revue ``Hot Night in Sochi'' or perhaps the latest hard-core magazine from Riga, Eroticon?

For one reason or another, Soviet President Mikhail Gorbachev has formed an anti-pornography commission to take ``urgent measures to protect public morals.'' And not a moment too soon. Dmitri Likhachev, an eminent scholar of ancient Russian literature, has warned that the proliferation of smut threatens the young with a ``spiritual Chernobyl.''

Gorbachev's glasnost has not been all Pasternak, Solzhenitsyn and other high-minded work. It has also meant - as a stroll through any subway station here reveals - the sale of bootleg copies of ``Emmanuelle''; a ``Sex-Hor-O-Log,'' which tells dogma-weary Capricorns when and if they can expect a little help from the

heavens in the potency department; and translations of the Kinsey Report and far racier handbooks. Even Komsomol, the Communist Party youth organization, shows soft-core videos for big rubles.

Even global diplomacy has the air of the burlesque hall. The other day, on the breakfast TV show ``120 Minutes,'' a cartoon showed the ``Soviet Union'' - in the person of a woman with a flimsy hammer-and-sickle brassiere - trying to seduce Uncle Sam. Curiously, she did not succeed.

The Bolsheviks did not start out with a prudish revolution. Alexandra Kollantai, a member of Lenin's inner circle, once said that to satisfy the sexual urge is as natural as quenching one's thirst with a glass of water. But with the years, sexuality all but disappeared from Soviet literature and popular culture, and the symbols of motherland were the puritanical Komsomol types who could have been mistaken for the grim crowds in ``The Scarlet Letter.''

But as the censors began losing their jobs in recent years, sex returned to page, screen and subway station. Soviet publishers brought out an edition of Vladimir Nabokov's ``Lolita.'' And once the secrets of the Stalin era and Kremlin politics were no longer taboo, the underground press turned instead to sex. Even Soviet television, which used to be best known for its intense interest in tractor combines, is now a good deal more permissive than the American networks.

Alexander Vasiliets, for one, is not amused. A deputy in the Supreme Soviet, Vasiliets told a session of the Legislature this week, ``Our country is being overwhelmed by an avalanche of sex, violence and other such slime. The junk is being sold everywhere in the streets and underpasses.''

At one point, Vasiliets turned to Leonid Kravchenko, the head of Soviet television, and said, ``Comrade Kravchenko, was this what you were appointed for?''

In an order published in a recent issue of the Communist Party paper Pravda, Gorbachev said that to stem the proliferation of ``pornography, pseudo-medical literature, erotic videos and similar publications,'' the new committee should be guided by the experience of other countries.

Presumably, the panel, led by actor and culture minister Nikolai Gubenko, will know smut when it sees it. Like the Supreme Court before him, Gubenko will be faced with the eternally vexing distinction between erotica and pornography. What will he say when faced with, say, ``Russian Beauty,'' the new novel by Viktor Yerofeyev? The main character is a prostitute who sleeps with men, women and diplomats. The book opens with a gynecological examination of great detail. At the Frankfurt book fair this year, ``Russian Beauty'' was one of the hot properties, and there are already translations available in French, German and many other languages.

Nikolai Smirnov, a doctor at Moscow's Institute for Sexual Culture, said in an interview that Soviet cities can no longer afford to ``let all this stuff appear in every corner of the city. We need restrictions, like in the West. We'd be a lot better off if there were `sex shops' and age limits and all the rest.''

Smirnov said he was especially distressed that ``charlatans are offering advice on sex, and they just don't know what they're talking about.''