"The Last Married Couple in America" is alarmist force about the vulnerability of modern marriage -- an institution which might as well be put out of its misery if it were practiced only by the numbskulls invented by screenwriter John Herman Shaner.

Southern California comedies like "Divorce American Style' and the Paul Mazursky tandem of "Bob & Carol & Ted & Alice" and Blume in Love' were vastly superior as both light entertainment and social documentation. Even now they would seem like funnier, timelier depictions of upper-middle-class marriage threatened by cultural change, including the self-centered discontent nurtured by status and affluence.

Shabbily contrived and couple cently smutty, "Couple" is interesting only as a reflection of southern california sociopathology, an example of the have-our-cake-and-eat-it syndrome that remains indispensable to Hollywood hacks. The film pretends to be fashionably knowing and up-to-date, but its frame of reference is ridiculous and its value system crassly conventional.

George Segal and Natalie Wood, who played Blume and Carol for Mazursky, are supposed to embody the exemplary, if sorely strained, marital alliance in "Couple." He is Jeff Thompson, successful architect. She is Mari Thompson, who raises their three boys while still finding time to pursue her sculpting at home. To document this talent, there's a sequence in which Wood is discovered contemplating a slab of inexpressive rock with chisel in hand and one cute little smudge of dust on her shirt.

The devoted Thompsons are introduced as one of four couples who gather on weekends to picnic and play touch football. Suddenly playing catch by themselves one weekend, the Thompsons are understandably distressed at the epidemic of separations within their set. At this point, Shaner resorts to sabotage -- of a peculiarly sleazy and unconvincing sort -- in order to drive his compatible couple temporarily apart.

The famished new Valerie Harper -- some austere regimen has transformed her into a bronzed, skeletal shadow of herself -- enters as a divorced snake-in-the-grass named Barbara. A former schoolmate and treacherous old friend of Mari, Barbara boasts of her new-found promiscuity and illustrates it by throwing herself at Jeff, who appears to let himself be seduced for the screenwriter's convenience. Even the little things --like a slight double-chin -- give Wood a substantial romantic advantage over Harper in this picture.

His affair with Barbara earns Jeff a glummer disposition and a case of veneral disease The awful truth comes out as the payoff to a miserably facetious scene where Jeff, complaining of "strep throat," escorts Mari to the doctor for a preventive shot. There they discover Barbara also awaiting treatment for the ubiquitous strep. To make two-and-two easier for slow-on-the-uptake Mari, the adulterers proceed to bicker candidly while she's still in earshot.

The Thompsons, allegedly the last outpost of stability and contentment of the American marital front, join the ranks of the separated. Mari, demonstrating a common sense and selfrespect that somehow eluged her husband band, brushes off a sexually opportunistic divorce lawyer played by Bob Dishy, who makes this would-be comical creep seem rather more menacing than laughable.

A puzzling gap in continuity finds the separated Thompsons consorting with youngsters before meeting at an anniversary party for a mondo weirdo couple: Dom Deluise as a blithe plumber-turned-porno-actor and Charlene Ryan as his discomaniacal happy-hooker wife. An encounter with an insinuating swinger, played very wittily by Mark Lonow, provides the Thompsons with an excuse for picking up the pieces.

Every so often Shanar seems to detect an authentic comic bleat of affluent discontent. For example, Richard Benjamin, as one of Segal's divorced cronies, is quite funny voicing laments like the following non sequitur: "Everything's so confused! Police strikes, Women's Lib, condominiums...."

Every crucial motivation or plot development in the script seems compromised by the sort of desperate dirty-mindedness that is now a cliche of television situation comedy. Verbally, "The Last Married Couple in American" is differentiated from "Three's Company" or "The Ropers" or "Soap" only by the augmented profanity which earns it an R rating. Visually there's no difference at all.

The climactic party sequence betrays the tiresome hypocrisy of all these halfhearted comedies of infidelity -- "Same Time, Next Year" is probably the most successful of recent years -- in which adultry functions as a soft-core tease, invoked to confirm the sanctity of some synthetically "happy" marriage. The Thompsons residence is invaded by a horde of oversexed freaks, the better to affirm their underlying devotion to each other.

The circumstances make their reconciliation look as absurb as their breakup, this scene seems to reflect a deep-set Hollywood compulsion: to appease decadents and squares simultaneously.