Have theatergoers become so bored and desensitized they can no longer distinguish between obscence titilation and entertainment?

In the opening spiel in "Dancin'," the Bob Fosse musical that opened to predominantly favorable but mixed reviews and is packing them into the Broadhurst in New York City, an offstage emcee boasts that the show has the daring to consist of nothing but dancing, without the usual props of story or theme.

It is true enough that the show crackles with visceral electricity - Fosse's forte - and that the dancers, with sleek, leggy, alluring Ann Reinking in the No. 1 spot, toss off showers of sparks.

But the real content of the show has less to do with dancing than with sanitized pornography, the kind of packaged sleaze Broadway has made into a specialized industry over the past two decades. The ability of an audience to enjoy such material depends on a willingness to equate theatricalized smut with "sophistication."

The issue here isn't erotic candor - the subject of "Dancin'" isn't sex, but prurience. The question it raises beneath its facile show-biz exterior is, how much raunch can you get away with without going hardcore.

The answer would seem to be quite a bit indeed these days, when you consider the overall reaction of press and public.

At the matinee I attended, the audience was nine-tenths a theater party crowd, averaging well over 50 in age. Twenty years ago this same kind of touristy assemblage, of like vintage and dress, would have been screaming for the constabulary midway through the show's first number. These people weren't exactly shouting themselves hoarse with bravos, but they weren't bolting for the exits either. The difference, one suspects, isn't so much a sign of enlightenment in the boondocks as it is evidence of a general erosion of our sense of propriety and taste.

A widespread critical complaint about "Dancin'," even among some of the show's admirers, was that in the absence of plot or unifying motif there is nothing to make the show cohere - the numbers don't hang together.

This is undoubtedly true, but maybe it's also beside the point. Most ballet performances and modern dance recitals, after all, can claim no more programmatic cohesion than "Dancin'," and although, like "Dancin'," they consist of unrelated dance "numbers" in frequently random assortment, there's rarely any balking on this account. One wonders whether anybody would be bothered by the fragmentary revue format of "Dancin'" if the choregraphy weren't so thin and superficial, so unremittingly frenetic in tempo and obsessively singletrack in tone.

"Dancin'," like the young stud in the proverbial maiden's lament, has just one thing on its mind. In "A Chorus Line," the model which Fosse clearly hoped would be one-upped by his own effort, it's the script and lyrics which take audiences by surprise with their unexpected and gratuitous vulgarity. In "Dancin'," it's the choreography which continually surpasses itself in crudity.

The "Dream Barre" number uses a ballet class (to ripped-off Bach) as the pretext for the graphic ravishing of a ballerina. In "Percussion," the jungle atmosphere and "primitive" music (including a wretchedly performed version of Varese's "Ionization") set loose an orgy of thrusting groins."Benny's Number" makes a jazz club the scene of a menage-a-trois. In "Joint Endeavor," the most flagrant instance, a daisy chain of drug-tripping swingers wriggles through an entire catalog of copulatory positions and techniques.

In Fosse's choreography, hands roam all over clutching and fonding; legs scissor apart and lock together around torsos or heads; faces are forever turning up between thighs. And Fosse's movement idiom, in which everything undulates - neck, shoulders, abdomen, hips - has become the apotheosis of the bump and grind.

The word "pornographic," moreover, is more than loosely applicable to dances so rampant with sadomasochistic imagery, the degradation of females and the "cool" refusal of sentiment.

There are several numbers in "Dancin'," to be sure, which are in no way lubricious. With the exception of "Dancin' Man," though - a bouquet to '30s musicals - they are disappointing for other reasons, and in at least one case, the =Bojangles" routine, offensive on other grounds (gross racial stereogyping, in this instance).

The annals of the musical stage abound in examples, from "One Touch of Venus" to "Silk Stockings" to "Cabaret," which show that it's possible to be sexy without being disgusting at the same time. The pity about "Dancin'" is that Fosse didn't have to be gross to raise the show's temperature; it could have been hot stuff on the strength of his talent alone, not to mention the sinky charms of the dancers.