"A Little Sex" opens with a gargantuan close-up of a match head igniting and being applied to the tip of a cigarette. The cigarette burns down to the filter, causing an elongated and hideously enlarged ash to collapse at the fadeout. If it weren't for the lettering of the credits and the sound of Melissa Manchester torching a theme song, suggestively titled "Your Place or Mine," this spectacle might be mistaken for a brilliant abstract short commissioned by the American Cancer Society to discourage smoking.

The credit teaser is meant to be symbolic all right. We learn that the morose hero, Michael Donovan, a director of TV commercials played by Tim Matheson, hasn't been able to break a smoking habit. It's one of novice screenwriter Robert De Laurentis' little conceits to equate this weakness with Michael's inability to swear off a glamorously loose sex life after he becomes a bridegroom. Exploiting the "theme" of compulsive infidelity with a coy, hypocritical obtuseness that defies toleration even in the context of a trifling romantic comedy, De Laurentis ensures a brief, droopy, ashen box-office future for "A Little Sex." Indeed, this is the kind of ignominious little loser that usually fails to pay for a theater's nightly electrical costs.

The hopelessly trivial nature of Michael's dilemma is summarized in the opening expository sequence, when he seeks advice from big brother Tommy, identified as a vet at the Central Park Zoo and portrayed with airy amusement by Edward Herrmann, who is able to breeze through the enveloping drivel by monopolizing the droll, worldly wise lines. Meanwhile poor Matheson, a blithe charmer himself not so long ago as the ringleader in "National Lampoon's Animal House," must scowl and fret to reflect a melancholy, conscience-stricken frame of mind.

"Why," Michael asks, "can't I have a cup of coffee with a woman without ending up in bed with her?" Although Michael's profession exposes him to an abnormal concentration of beautiful women, there are limits--to credibility, if nothing else. Since Matheson doesn't betray an ironic smile while speaking this hilariously hyperbolic line, one gathers that it's supposed to be taken seriously, a self-evident impossibility. A cheerfully libidinous rascal in "Animal House," Matheson has suddenly lost his blooming good humor in "A Little Sex," and the change doesn't suit him.

Moreover, Michael and his bride, Katherine, a parochial-school teacher played by Kate Capshaw, whose most notable attribute is a head of bouncy brown curls, present such an infantile image of young marrieds that you quickly cease to care whether they make a go of things, break up, swing from the rafters or vanish from the face of the screen. The earnest note Michael sounds in conversation with his brother somehow disappears when he encounters Katherine. They "meet cute" by carrying on a bogus racy conversation within earshot of supposedly scandalized innocent bystanders at a grocery store. Then they chase each other home to bed.

Unfortunately, the script prattles on. Michael and Katherine indulge in another exhibitionistic prank while shopping for a bed and then tie the knot. Still surrounded by voluptuous temptation, Michael weakens. Kate finds him in a compromising position at the office one evening. The situation is doubly poignant because she, acting upon the zany advice of a colleague, has decided to surprise him by showing up with nothing on under her raincoat. The funniest line in the show grows out of this Embarrassing Moment, although it's played with agonizing seriousness. Running after his outraged spouse while still buckling his trousers, Michael pleads, "It's not what you think!"

After a period of disgrace, which also permits the injured bride to turn the tables by sampling an adulterous snack herself, a contrite Michael is given the opportunity to make amends, although it appears he may have reformed at the cost of permanent disadvantage on the conjugal front. Michael reaches the conclusion that "All the one-night stands aren't worth one minute with the person you love," while Katherine, who found her one-night stand pretty tingly, makes peace on a suspiciously conditional note: "As good as that kind of sex is, it's not worth giving up a marriage for." That's some meeting of the minds.

I realize I'm a strait-laced spoilsport about these matters, but it's my impression that fidelity is not a herculean achievement for men who are sincerely in love with their wives, couldn't imagine living without them and have convictions about the sanctity of marriage. "A Little Sex" asks us to derive both sniggery amusement and spongy empathy from the notion that Michael is profoundly stuck on Katherine but can't summon the will power to resist fooling around while still a newlywed. It would require an ingenious humorist to make something substantial out of so delapidated a premise. Lacking that resource, "A Little Sex" goes up in smoke while attempting to blow smoke at the audience.