BACK WHEN IT was all blue skies for blue movies, when people were taking their mothers to see "Deep Throat," fans of the genre would innocently daydream about how fulfilling, how terribly exciting it would be when a major, world-class director made a hard-core pornographic film. Yet if Nagisa Oshima's "In The Realm of the Senses" is any guide, we didn't know when we were well off.

This, it must be admitted, is a minority opinion. When "Senses" premiered at last year's Cannes Film Festival, the crush outside the theater two hours beforehand was worse than a New York subway nightmare, completely blocking traffic on one of the city's main streets and causing more screams of anguish than of esctasy.

Since then, "Senses" has appeared in seven more festivals, has opened to record receipts in Los Angeles, San Francisco and Chicago, and has inspired reviews modestly calling it "the most thoughtful work of art on eroticism yet created," "the most convicing and compelling portrait of an over-powering sexual and romantic obsession that I have seen on the screen" and so on.

Yet though it feel features erection, ejaculation and all the others flora and fauna of pornography. "Senses" is as unstimulating, anti-erotic a film as can be imagined, a slow, heavy, claustrophobic work perfectly in line with Oshima's other elliptical, ritualistic films but hardly anyone's idea of a good time.

Based on a historical incident, "Senses" tells the story of Sada and Kichizo, a Japanese fun couple of some 40 years back who can't keep their hands off each other, even for a minute. After an hour and a half of dialogue like "It's marvelous, I'm going to kill you," Sada in fact does kill Kichizo and castrates him for good measure. Can this marriage be saved?

Oshima as a legitimate film artist has of course the right to be as obsessive, unpleasant and tiresome as his little heart desires, but the really interesting questions are why should a film like this become so popular and how does it fit in with what Oshima's less pretigious colleagues are doing in the world of hard-core.

In many ways, "Senses" appears to be the "I Am Curious Yellow" of the 1970s. For while experience, not to mention box office grosses, makes it obvious that the American public has no small appetite for films of a sexual nature, just any old film can't satisfy. It has to be something sanctioned.

If there are enough worshipful reviews around assuring everyone that this is a serious tie , borring film, something you can go to without blushing at the box office, people will turn out in hoards. With enough pious words as protection, no one feed even a tiny bit ashamed for lustful thoughts.

And as films like "I Am Curious," the French "Exhibition," "Bertolucci's" Last Tango in Paris" and "Senses" prove, this sanction is more likely to go to films by foreign directors, obviously much more "adult" folks and more perfectly at home with earnest, weighty topics like sex than poor naive Americans could ever hope to be.

Even more distributing is the way these films inevitably turn out to be terribly joyless affairs. In this "Senses" follows the unfortunate lead more down-to-earth pornography has taken in recent years, becoming increasingly preoccupied with unhappy sado-masochistic fantasies and/or nightmares.

One is not asking here for pastoral romps that look like X-rated Salem commercials, bur rather something on the line of the Donald Sutherland Julie Christie love scene in "Don't Look Now:" more or less conventionally normal people making more or less conventionally normal love in an explicit, erotic way.

What is wanted is more of those moments, bright electric moments, when it all works, when the film on the screen, by some accident of luck and talent, transcends itself, becoming simultaneously drama, eroticism and cinema and producing a sexual sequence of enormous emotional intensity.

Unfortunately, what we get with "In the Realm of the Senses" is an insidious that nonetheless extremely moralistic view of the end result of sexual esctasy, a kind of Puritanical punishment-fits-the-crime that tells you, just as all those pseudo-serious, pseudo-erotic foreign films have before it, that if you enjoy uourself you're doing to really have to pay for it. For who knows what reason, this is obviously something Americnas have a great need to hear.