If the point needed proving, "La Cage aux Folles," a French-Italian production opening today at the K-B Janus, demonstrates that Europeans can fabricate a farce as crude as "Norman, Is That You?" on the supposedly hilarious premise of square straights encountering dizzy homosexuals. As a matter of fact, "La Cage aux Folles" appears to be the French equivalent. According to harrowing reports, the original play by Jean Poiret has been running seven consecutive years in Paris.

The title refers to a chic cabaret in St. Tropez where female impersonators provide the floor show. The proprietor, Renato, a dignified, composed, mildly flouncy queen played by Ugo Tognazzi, resides in an adjacent apartment with his lover of 20 years, Michel Serrault as the lisping, fluttery, temperamental Albin, who headlines at the club under the name of Zaza.

Renato's son, Laurent who has been brought up by Renato and Albin, announces his engagement to a nice, dutiful bourgeois girl, Andrea, whose straitlaced parents, a domineering reactionary politician and his wife, insist of meeting the prospective groom's family.

Having contrived this feeble social crisis and struggled to keep it afloat with stale, excruciating sex jokes, the screenwriters (Poiret included) arrange an implausible climactic dinner party at the abode of the homosexuals, who bend over backwards to stump the squares by toning down the interior decoration and suppressing telltale mannerisms. The comical confusion theoretically crests when Laurent's real mother arrives soon after Albin has tried to pass himself off as mom in drag and reporters turn up to corner the politician in a scandalous setting.

What public sustains such oppressive stumblebum facetiousness? On the face of it the material should insult both self-respecting heterosexuals and homosexuals. It's difficult to decide which set of potential in-laws behaves in the more incredibly stupid fashion.

Although the up-tight, moralistic straights are positioned to serve as the climactic butts of the joke, the middle-aged gays are exploited as an ongoing behavioral joke.

Serrault, who made his movie debut 25 years ago in "Diabolique," portrays Albin with all the mincing stops out. He swishes everywhere he goes, emits four-alarm squeals of surprise or delight and throws foot-stamping, bottle-throwing tantrums. Although Renato is more restrained, he too can't lift a cup without elevating a prissy pinky or hear a champagne cork pop without shrilly exclaiming, "Whoooo!"

Renato and Albin enter and depart squabbling like old sillies. The first dispute is evidently resolved when Albin chirps, "Go ahead, hit me if you want" and then turns up in the next scene sporting a shiner. Poiret's taste in little surprises is not ingratiating. For example, the identification of Laurent is lewdly delayed, inviting the audience to take him for a kid Renato is trying to make rather than Renato's own child.

Director Edouard Molinaro has evidently failed to devise a playing rhythm to compensate for whatever farcical tempo the material enjoyed on the stage. The lurching, stop-and-go disorganization of the movie sequences puts a ragged edge on an already tacky piece of merchandise. CAPTION: Picture, Michel Serrault