LEAD: IN their time, almost all forms of popular American music and dancing, from the foxtrot and the tango through rock-and-roll and all of its variations, have scandalized the members of an older generation, whose own sexuality had earlier been liberated by tamer means. As music, lyrics and dance steps have become more and more sexually explicit, fathers and mothers from coast to coast have felt alienated, and worried that pop music was leading their children straight to hell.
IN their time, almost all forms of popular American music and dancing, from the foxtrot and the tango through rock-and-roll and all of its variations, have scandalized the members of an older generation, whose own sexuality had earlier been liberated by tamer means. As music, lyrics and dance steps have become more and more sexually explicit, fathers and mothers from coast to coast have felt alienated, and worried that pop music was leading their children straight to hell. As it was with the bunny hug, danced to a ragtime tune in 1910, so is it today when Madonna sings ''Papa Don't Preach.''
This culture generation gap has produced its own Hollywood genre. Most of these films have been quickies on the order of ''Don't Knock the Rock'' (1957) and ''Twist Around the Clock'' (1962), but there have occasionally been more ambitious if not much better films (Herbert Ross's ''Footloose,'' 1984). Though music is the subject of each film, sex is the subtext. In the final reel, generations reconcile; initially stuffy oldsters end up rocking, rolling or twisting the night away, showing the young that, though creaky of joint and infirm of body, they can still do ''it.''
''It'' is also the subject of ''Dirty Dancing,'' which opens today at the National and other theaters.
''Dirty Dancing'' is a nicely bittersweet genre movie set at Kellerman's Mountain House, a Grossinger's-like Catskills resort hotel, in the summer of 1963. President Kennedy was still alive, America's stake in Vietnam had not yet become divisive, and socially conscious young people were going on freedom marches in the South and joining the Peace Corps.
The film is about Frances Houseman (Jennifer Grey), nicknamed Baby, a pretty middle-class teen-ager who finds her adult identity in her first love affair - with Johnny Castle (Patrick Swayze), the hotel's dance instructor. For a girl of Baby's conventionally liberal, Jewish background (she's planning to study ''third world economics'' in college), Johnny, a young man from the wrong side of the tracks, exemplifies the freedom expressed through a new and as yet socially unacceptable form of dancing. This ''dirty dancing,'' a phrase used only in the film's title, features a lot of steamy body contact and pelvic thrusts, which unleash emotions supposedly left withered by mambos and cha-cha-chas.
Taking a formula that is itself creaky of joint and infirm of body, Eleanor Bergstein, the writer, and Emile Ardolino, the director, have made an engaging pop-movie romance of somewhat more substance than one usually finds in summer movies designed for the young.
I suspect that one's responses to ''Dirty Dancing,'' to its period details, even to its state of mind, will depend on the associations one brings into the theater. What is undeniable, however, is a basic decency of feeling, shaped, in part, by the film's obligations to its optimistic genre.
Baby, as written by Miss Bergstein and played by Miss Grey (''Ferris Bueller's Day Off''), is no bubble-brained teen-ager, but a bright, inquisitive young woman who's on her way to being her own person. Miss Bergstein is much better on creating character than in re-imagining formula events. Baby's liberation comes through her forbidden association with the womanizing Johnny Castle, after his partner, Penny (Cynthia Rhodes), becomes pregnant and Baby agrees to substitute for her in a mambo demonstration at another hotel.
There's a really quite awful subplot about Penny's abortion, financed by money that Baby has borrowed from her conventionally liberal doctor-father, and about the arrogant young Ivy League fellow who is responsible for Penny's condition.
Given the limitations of his role, that of a poor but handsome sex-object abused by the rich women at Kellerman's Mountain House, Mr. Swayze is also good. He's even convincing when he must admit, in one of the film's lesser moments, that ''the reason people treat me like nothing is because I am nothing.'' He's at his best - as is the movie - when he's dancing.
The movie makes a lot of good use of period music, to which some not very evocative new songs have been added. The dancing itself, especially the dirty dancing, choreographed by Kenny Ortega, looks very contemporary, or, at least, as contemporary as ''Saturday Night Fever,'' but it has a drive and a pulse that give the film real excitement. Though the film takes place in 1963, just a year after the twist was all the rage, the twist itself seems already to have come and gone at Kellerman's Mountain House. The women's clothes also look surprisingly mid-80's and, in 1963, would anybody have used the term ''wimp''?
These anachronisms aren't especially important, except that Miss Bergstein has been so specific about the film's period. She seems to want ''Dirty Dancing'' to be seen as a fond goodbye to a comfortable, liberal American way of life before the country was radicalized by the assassination of President Kennedy and by the increasingly bitter anti-Vietnam War movement. That's loading a small movie with rather more than it can carry without a lot of highly detailed program notes.
''Dirty Dancing'' works best when it's most direct and unpretentious. It has the kind of sweet simplicity that somehow always eludes John Hughes (''Sixteen Candles,'' ''Pretty in Pink,'' ''Ferris Bueller's Day Off''). Mr. Ardolino, whose background is in theater and in television dance films, doesn't clutter the film with extraneous, sentimental detail, nor even with too much colorful (and familiar) detail about life in your usual Catskill resort hotel. He also obtains excellent performances from his cast, which, in addition to Miss Grey, Mr. Swayze and Miss Rhodes, includes Jerry Orbach, as Baby's father; Jack Weston, as the owner of Kellerman's, and Lonny Price, who is especially funny as Mr. Weston's arrogant, wimpish grandson.
''Dirty Dancing,'' which has been rated PG-13 (''Special Parental Guidance Suggested for Those Younger Than 13''), includes some vulgar language and a lot of dancing that is comparatively erotic. Watch Your Step DIRTY DANCING, directed by Emile Ardolino; written by Eleanor Bergstein; director of photography, Jeff Jur; choreography by Kenny Ortega; music by John Morris; edited by Peter C. Frank; produced by Linda Gottlieb and Ms. Bergstein; released by Vestron Pictures. At Gemini Twin, Second Avenue at 64th Street; National Twin, Broadway at 44th Street; 86th Street East Twin, near Second Avenue; 57th Street Playhouse, at Avenue of the Americas; 23d Street West Triplex, near Eighth Avenue; Loews 84th Street Six, at Broadway. Running time: 86 minutes. This film is rated PG-13.
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