Ah, these French. So much charm. So amusing. So naughty! Such, at least, is the image of Gallic insouciance and erotic mischief director Yves Robert would like to foist upon us in his latest comedy, "Pardon Mon Affaire," currently ensconced at the K-B Cerberus 1.

The picture works so hard to project itself as a fluffy little sex farce it almost makes one feel like a curmudgeon not to be enjoying it more. It's not bad, really, just utterly trifling, and very thin even for its feather-weight ambitions. There are half a dozen or so genuinely rib-tickling strokes, and minor smiles along the way. But what do you do in between, waiting for the plot to sidle up to a punch line?

The beginning is promising - too promising, as it turns out. The camera retreats from the face of the hero until we realize he's perched on a building ledge overlooking the Arc de Triomphe, wearing nothing but a bathrobe, caught with his pants down, as the saying goes.

The flashback which follows puts Etienne - he describes himself as an "average family man" - back in his normal setting, behind a desk in the ministry of finance. One morning, parking his car in the building garage, he spies a slender young thing whose skirt gets blown over her head, and he's off to macho fantasyland. The remainder of the movie is a helter-skelter account of his campaign to achieve carnal knowledge of his dream girl, and the lengths of deceit, prevarication and idiocy he's prepared to go to in pursuit of this objective.

Fate, however, and Etienne's own clumsy maneuvering, keep getting in the way. Before he makes it to home plate in the last reel, just about everything seems to go wrong, and once he does, he's more in the soup than ever. But does this lead him to see the error of his ways? Not on your life, that wouldn't be French, now, would it? Instead, as the picture fades from the screen, he's cooking up his next conquest.

Etienne is not alone in his misbehavior. His wife, to be sure, who has gone back to school at 41 to get a degree, manages to resist the advances of a 17-year-old who's obsessed with her figure. But every other character in the film is a cheater - it's a sort of 'Cosi fan tutte" transposed to contemporary Paris. Etienne has three buddies he plays tennis with, one of them a homosexual, and they're all doing it. So is the wife of one of the pals. So is Charlotte, the model who is the object of Etienne's lust. In modern-day France, marriage, Robert would have us believe, is honored far more in the breach than in the observance.

The trouble is, Jean-Loup Dabadie's screenplay extracts so little daring wit out of all this dalliance - there's more racy humor in half an episode of "Mary Hartman, Mary Hartman" than in the entire film.

As far as it goes, it's well done. Jean Rochefort is expertly nonchalant as the self-deluding Etienne. Victor Lanoux, Guy Bedos and Claude Brasseur are aptly cast as his roving comrades and Daniele Delorme as his wife. The vacantly pretty Anny Dupcrey is sufficiently alluring as the elusive prey. Robert, who's also given us "Alexander," "The Tall Blond Man With One Black Shoe" and "Salut L'Artiste," is an old hand at this sort of thing and carries it off with reasonable efficiency, if no great sparkle. The inconsequentiality of Victor Cosma's diddly music seems to suit the subject.