ADULTERY HAS always enjoyed general acceptance in European cinema, as part and parcel of a culture in which marriage a la mode has been a social and literary convention for centuries. It took the New World, if not to invent the idea of romantic marriage, then at least to invest it with the freedoms of choice available in our democratic society. And it took Hollywood to canonize it in propaganda for the American Way of Life.

Marital infidelity, if examined at all in the American cinema, was seen largely as the pastime of monied socialities or decadent European aristocrats -- or as something people did only on their vacations.

Now, after the sexual revolution has at least superficially shattered every other taboo, Hollywood has finally discovered Adultery for Everyperson. Audiences can look forward this year to a glut of domestic versions of the kind of domestic come dies in which the French film industry has excelled for 40 years:

"Last Married Couple" is directed for Universal by Gil Cates from the screen-play by John Shaner. George Segal and Natalie Wood are the happily married pair who, through media and peer-group pressure, are encouraged to "swing" -- with disastrous results.

"Two Plus Two" will be produced by Edgar Scherick for Marble Arch Productions from Larry Gelbart's script. It centers on a Long Island doctor whose 18-year marriage is drifting apart. He falls in love with the comely wife of an Italian gardener, both have a deep need and affection for their strongly structured families, which is intended as the real focus of the film.

"Loving Couples," being directed by Jack Smight and produced by Renee Valente for Time-Life, is a woman's view of the same situation, Two doctors (Shirley MacLaine and James Coburn) are married to each other. The passion of work cools for MacLaine at 40, and she falls for an overly romantic young man (Stephen Collins), whose girlfriend (Susan Sarandon) then seduces Coburn. Through the interplay of the "loving couples," husband and wife finally realize the value of their relationship.

In "Switching" (to be directed by Alan Parker for 20th Century-Fox from Bo Goldman's script), the truth, as in "Manhattan," comes out of the mouths of babes. George, married to Faith, achieves success suddenly in midlife, and wants to shed his old wife along with his old skin. He has an affair. Faith counters with one of her own. It takes their 14-year-old daughter to illuminate that what is wrong in the marriage is George's latent fear of his wife.

"Middle-Age Crazy," directed by John Trent, stars Bruce Dern as a good ol' Texas contractor who has a crisis approaching the age of 40 -- aggravated by his father's death and wife Ann-Margret's sexual demands. He takes off with a Texas cowgirl but can't handle sex without rules and goes back to the fold.

Then there are the menages a trois:

Though neither contemporary nor comedic, the just-released "Heart Beat" (directed by John Byrum from his own screenplay and starring Nick Nolte, Sissy Spacek and John Heard) deserves mention as its focus in the three-way relationship between "Beat" novelist Jack Kerouac and Neil and Carolyn Cassidy.

"Just Tell Me What You Want" (directed by Sidney Lumet for Warners) stars Ali MacGraw as a TV executive who must choose between the charming-but-manipulative older man (Alan King) who made her in both the professional and the romantic sense, and the young writer whom she marries secretly to get away from him. The twist is that it is her lover who plays the father role in her life.

"Willie and Phil," written and directed for Fox by Paul Mazursky, stars Ray Sharkey, Margot Kidder and Michael Ontkean. It is an "homage" to "Jules et Jim," Francois Truffaut's lambent World War I study of male-male friendship expressed penultimately through the love of the same woman. But, while "Jules et Jim" explores the darker angles of sexual ambiguity and ends in fact with the deaths of two of the trinity, Willie and Phil separate unscarred at the end of the 1970s, each to his own destiny.

Three years ago, these films would have been relegated to TV by studio executives as too "soft" (an adjective that in Hollywood means either "nonplot-action-oriented" or "feminine in thinking"). But now, executive cite a variety of reasons for the change: a disenchantment with the emotional shallowness of special-effects movies as typified in the works of Steven Spielberg and Ridley Scott; the desire of stars such as Jill Clayburgh and Burt Reynolds to sink their teeth into "real dramatic roles, and the realization that there's big box office in such European films as "Cousin, Cousine" and "Pardon Mon Affaires 1 and 2," to say nothing of runaway American hits like "An Unmarried Woman" and "Manhattan."

"Three years ago people weren't sure what kind of movie fare they wanted," says Renee Valente, the first woman vice president of Screen Gems. "People today are interested in a realistic escaptism, with the hopeful feeling that relationships can work."

Along with a divorce rate approaching 50 percent and television's increasing treatment of separation, one-parent households and ever more explicit personal relationships, another important factor is that the sexual revolution is no longer a novelty. Even feminist concerns have even crept into the traditionally conservative film industry. But why the focus on marriage?

"People are reaching for a family consciousness," says Valente. "What happened in the past 10 years has made people more aware of what values should be. Both men and women are more fearful. If a woman is career-oriented, what kind of relationship can a man expect from her?"

"'Switching is about terrible problems between men and women that seem to be surfacing today but which have been there for years," says author Bo Goldman, who co-wrote the screen-play for "One Flew Over The Cuckoo's Nest," a study of men in revolt against a female authority figure. Interestingly, "Switching" is the only one of the films in question to examine the fear and resentment in the mothering relationship that exists between wives and husbands.

The issues, according to Goldman, are really men, women and America: "There is such enormous pressure on men in this country to succeed, but once they do, they wonder why they feel so empty. People use a lot of buzz words like 'alienation,' but I don't think they mean much. Everybody wants change, but nothing lasts. 'Switching' is about verities, truths that I think people want to go back to, marriage as an institution in the best sense, that it stands for something, that it's here to stay."

The "new trend" in this year's movies rejuvenates an old tradition. In the late '20s and early '30s the studios, filled with brilliant refugees from Europe, turned out scores of highly sophisticated comedies-of-the-sexes. Ernst Lubitsch, for example, was imported by RKO specifically for his expertise in that area. He was, however, carefully kept within the limits of the Hays Code, which went so far as to preserve the sanctity of marriage by splitting the connubial bed into twins. (In 1930, the Motion Picture Productions of America's self-censoring office, directed by Will Hays, set up the Code of Production, later stringently enforced after pressure by the Legion of Decency).

Lubitsch's films were famous for innuendo. He also was known for casting primarily European stars, such as the sultry Pola Negri. The inference was that American audiences could get their dollop of sex through the use of European talent, but that the image of American stars had to be kept untarnished. A very few, like Kansas-born Louise Brooks, who radiated amorality, cross-migrated to Europe.

The "Golden Era" of the European filmmaker in Hollywood ended abruptly with World War II as the studios -- pressed to confirm the way of life for which Americans were fighting -- distilled a quintessential propaganda into their vision of men, women and the nuclear family. Even after victory, the intense moral surveillance of the McCarthy era seemed to freeze these attitudes. While premarital relationships continued to provide the obligatory "love-interest," postmarital relationships were relegated either to sitcoms in which the degree of sexual ignorance was matched only by that of sexual jealousy -- or to the murky underground of film noir in which adultery, practiced by irredeemable characters, met with inevitable and deserved punishment.

Billy wilder elicited howls of outrage from every quarter when he deliberately satirized the sexual immaturity of the period in "Kiss Me, Stupid." Adapted from an Italian play, "Kiss Me" equates American notions of love with American notions of property values in the story of an ambitious songwriter who, in order to avoid lending his luscious wife to Dean Martin, pays the town whore to play her part.

In the '60s the youth-boom exploded, burying marriage even deeper as a subject. If you wanted the sordid reality of mature relationships, you still had to sneak out to your local arthouse. In the '70s the whiz-kids took over. In general, personal relationships on the screen (excepting the work of marvericks such as Woody Allen, Robert Altman, Francis Coppola and Paul Mazursky) had been notable chiefly for their absence, from B-movie "homages" that ignore character for technique, to ever more sophisticated ways of depicting mayhem on the screen.

Now marriage is back and adultery is on the screen. And what effect the new movies' spirit of "realistic escapism" will have on the vast and disturbing problems afflicting American couples remains to be seen. CAPTION: Illustration, "Lights! Camera! Love!"; Copyright (c) , 1969, The New Yorker Magazine, Inc. by Koren; Picture, George Segal and Natalie Wood in "The Last Married Couple in America."