WITH ALL THE WATCHDOG groups delivering their alerts on violence in the media, perhaps it’s time for a sex alert.You see, there’s a nasty little movie out there that’s starting to do great business around the country and it’s — hold your breath — unrated. It didn’t even qualify for that dreaded invention, the NC-17 — or at least it didn’t want to. The movie is called “Y tu mama tambien,” and it happens to be the sharpest, most original movie released thus far this year. But it’s also a vivid reminder of the hypocrisies of the system. A year ago at this time, the Mexican-made movie was shown almost furtively to distributors in Cannes (no, it didn’t qualify for competition, nor was it officially screened for the “market”). The film scared the hell out of most of its viewers. “I loved the movie, but my company won’t go near it because of the ratings problem,” said the acquisitions chief of one mini-major. Mind you, the film was already a smash hit in its home country, but here was the rub: It showed people having sex. What’s worse, enjoying it. It even had a couple of fleeting shots of that ultimate no-no — a penis. FOR THE RECORD, “Y tu mama tambien” happens to be at once a shrewd political satire as well as a movie about sexual awakening. Directed by Alfonso Cuaron, who shot the widely praised family movie “A Little Princess,” it aims some delicious jabs at Mexico’s wealthy hierarchs. At one stiff social gathering, it is pointed out that there are more bodyguards present than guests — and they all look interchangeable. But this is also a road movie about two teenagers and a woman who has just left her unfaithful husband. Their trek across the Mexican countryside is periodically halted at armed barricades manned by soldiers who have no idea what or why they’re barricading. Despite the fact that it’s both funny and commercial and would likely score with both the arthouse and Hispanic audiences, “Y tu mama tambien” was spurned by majors and minors alike. Parenthetically, it also wasn’t helped by a tepid review in Variety a year ago written by a former stringer in Mexico City. The movie finally was picked up by an energetic little outfit called IFC Films, whose marketing and distribution maven is Bob Berney. As part of the Bravo networks, the New York-based IFC has created its own low-key synergy with its cable outlets and its IFCRant magazine. This is hardly AOL Time Warner stuff, but it gets the job done — witness the fact that its little Mexican pickup seems on the road to doing $15 million or better in the U.S. And without a rating. MIND YOU, BERNEY took a stab at getting an R, working with the MPAA ratings gurus to snip the sex scenes. The trouble was that the laughs were lost along with the sex, so the snips were restored and the movie went out in its original form. This was also the course previously followed by movies like “Fat Girl,” “Intimacy” and “Requiem for a Dream.” “Requiem” ended up doing far better than, say, “L.I.E.,” a film of considerable merit whose producers opted for an NC-17. It quickly disappeared from sight. The problem is simple: The NC-17 stigma gets a film banned by big theater chains; newspapers refuse to carry its ads. By contrast, the unrated “Y tu mama tambien” hasn’t met with any turndowns from theaters or newspapers. The MPAA won’t admit that its NC-17 experiment is a flop. While it encourages distributors to submit their films for ratings and mandates that member companies do so, the group insists that “the system is entirely voluntary.” Trouble is, it’s also entirely punitive. Meanwhile, Berney keeps expanding his rollout to new theaters and new cities, and customers keep coming. And the pic’s backers, including an obscure Mexican vitamin manufacturer named Jorge Vergara, are having the proverbial last laugh. As for IFC, it’s about to unfurl its next release, a somewhat tamer comedy called “My Big Fat Greek Wedding.” Its list of producers is slightly better known: Mr. and Mrs. Tom Hanks.