The very title of “Meet the Fockers” is sufficient warning that you shouldn’t go to it expecting a subtle, sophisticated comedy. That’s alright: its precursor, the 2000 “Meet the Parents,” was hardly high-toned entertainment, either. Still, the earlier picture’s scattershot script about a doofus would-be fiancé visiting his girlfriend’s ultra-intimidating father–an ex-CIA man, no less–had its share of dumb, harmless laughs. And it certainly didn’t descend to the level of crass vulgarity that’s pretty much the only note the sequel hits (or even aims for). The moral at the end of “Fockers” is that it’s always best to tell the truth, so let’s do that: This picture is just an avalanche of sniggering sexual humor masquerading as a family comedy. Unbelievably raunchy–it would make aficionados of old British sex farces or burlesque shows positively blanch, and exceeds most of the crassest teen movies in tawdriness–it’s especially repellent for two reasons. One is that it’s been granted a PG-13 rating although it easily warrants an R, and probably would have gotten one but for the fact that it’s got big studio backing behind it. The other is that it represents the latest screen pairing of two of our greatest actors–Robert De Niro and Dustin Hoffman (who previously shared credit in “Wag the Dog” and “Sleepers”); that it should occur in such a piece of smarmy drivel is unconscionable.

The set-up is as simple as in the first movie. Nurse Greg Focker (Ben Stiller) and fiancee Pam Byrnes (Teri Polo) are now living together and planning their wedding. But one hurdle to happiness remains: Greg has to introduce his soon-to-be father-in-law Jack (De Niro) to his parents Bernie (Hoffman) and Roz (Barbra Streisand). So he and the Byrnes family, including Jack’s wife Dina (Blythe Danner) and their young grandson Little Jack (twins Spencer and Bradley Pickren), board Jack’s gargantuan RV for a trip to Focker Isle near Coconut Grove, a thoroughly appropriate residence, as it turns out, since Bernie and Roz are outrageous characters, pre-advertised by Greg to Jack as a top lawyer and a doctor but actually a stay-at-home ex-lawyer and a seniors’ sex therapist, respectively. What passes for comedy–and the verb is used here in the sense that one would employ it with respect to kidney stones–derives from the clash between the happily hedonistic Fockers and the grimly serious Jack; the latter’s efforts to derail his daughter’s pending nuptials by investigating Greg’s past with the former family maid Isabel (Alanna Ubach), a gregarious Latina with a son named Jorge (Ray Santiano) whose mannerisms are suspiciously like Focker’s; and the predictable complications that ensue when Pam discloses that she’s pregnant and the couple tries to hurry up the wedding ceremony without letting Jack know why. A subplot involves Jack’s obsessive efforts to raise Little Jack (whose parents are curiously absent) according to a very strict regimen which, of course, Greg turns into chaos. The ultimate point of it all is that Jack learns to loosen up, in every conceivable way. That’s supposed to be a good thing.

Your reaction to the lowbrow sitcom-quality content of “Meet the Fockers” will depend not only on your tolerance of gross slapstick and suggestive innuendo–all of it italicized by the sledgehammer direction of Jay Roach–but on your ability to forgive some nearly iconic figures being trapped in the morass. After all, there’s no shock in watching Stiller descend to such coarseness–it’s pretty much his stock-in-trade, although Greg’s big speech toward the close (delivered, supposedly, under the influence of truth serum) is an embarrassingly unfunny bit even for him. It’s the indignities inflicted on De Niro and Hoffman that are truly appalling. Do we really want to see the former wearing a fake female breast as a device for feeding his grandson, or being “mounted” from the rear by Streisand as part of massage therapy on his injured back? Or Hoffman, his face covered in whipped cream, snuggling in bed beside Streisand, whose breasts are also awash in the goo, or talking about his character’s testicles? It’s degrading to viewers to be fed such material, but even sadder to watch great actors degrading themselves by performing it. (To be fair, De Niro and Hoffman seem to be good sports about it, with the latter in particular expending a lot of energy along the way. But somehow that just makes it all the more depressing.) And the abusively vulgar treatment doesn’t stop with the three stars. Little Jack, who’s portrayed as having the ability to “converse” with pre-vocal sign language (a much less amusing conceit that the subtitling of Sunny Baudelaire’s gurglings in the Lemnony Snicket movie), serves as nothing more than a reaction-shot pawn, playing the sort of part usually assigned to an audience-pleasing dog; and the piece de resistance involving him comes when Greg carelessly teaches him his first word–an obscenity which he then repeats endlessly. (The twins’ parents must be so proud.) And even then the makers must have been concerned that Little Jack’s presence couldn’t entirely make up for an absence of animals; they add a dog to the cat from the previous installment, and then proceed to construct such moments of comic mastery as having the cat flush the pooch down a toilet. In a movie like this, it seems, potty humor must include humans and their pets alike.

Understandably, the remainder of the cast walks through the picture looking rather astonished at what’s happening around them. Streisand opts for a standard-issue diva turn, flamboyantly exaggerating all her best-known mannerisms; it’s less a performance than a series of poses. Polo is sprightly but vacuous, and Danner was probably pleased that Dina Byrnes is so unostentatious a person, able quietly to recede into the background. Owen Wilson briefly reprises his role as Pam’s once-upon-a-time suitor from “Parents,” looking as dazed as most viewers are likely to be by the time he appears, and Santiago does a reasonably good Stiller impression as Greg’s presumed offspring. Then there’s Tim Blake Nelson, who pulls out all the Barney Fife stops as a cop who gets involved with Greg, Bernie and Jack in the last act. On the technical side the picture is adequate but nondescript.

The fact that “Meet the Fockers” is being released at Christmastime may mislead you into thinking that it’s a picture you can take the whole family to. But that couldn’t be further from the truth. Its relentlessly blue humor makes it highly unsuitable for children. And for discerning adults, too, since it’s also remarkably unfunny. In other words, you don’t have to be a prude to feel good about declining the invitation its title offers, but if you are, you’d better make a special effort to stay away.