The Farrelly brothers have finally gone and done it. They’ve become soft.

That’s right, soft as in tame and mild-mannered. The same guys who made every man and woman in America cross their legs during the excruciating zipper scene in “There’s Something About Mary” have bogged down in their own scatological mire for their latest project, “Say It Isn’t So.”

Ironically, the Farrellys have wussed out just when they’ve embraced one of the last remaining taboos. That sibling incest doesn’t inspire much dumb yuks can be taken as a good sign for those who feared that this good Earth would be annihilated by its own depravity. It’s not so good for fans of the filmmakers of modern-day men’s romantic comedy, in which bowlers with a hook for a hand and a room full of stalkers are formulas for outrageous buffoonery.

Now technically, the brothers are the producers; they let J.B. Rogers, the assistant director for their last four movies, sit in the director chair this time while first-time screenwriters Peter Gaulke and Gerry Swallow wrote the script. Still, this is branded as a product from the Farrelly factory, and ultimately they’re responsible for quality control.

In “Say It Isn’t So,” orphan Gilly Noble (Chris Klein) is an animal catcher so sensitive and chaste, his sexuality is the topic of dinner conversation at his boss’ house. He has developed a philosophy that “loneliness is what insures the propagation of the species (and) that is one hell of a milquetoast reason to hook up with someone for the rest of their lives.” Noble apparently derives this sentiment from his abandonment and the dysfunctional relationships he witnesses, like that horrifying dinner with the boss, his wife and two kids. The experience is “the kind of thing,” his boss Larry Falwell says later, “that makes you want to bludgeon your family and go on a three-day drinking spree.”

Noble doesn’t let loneliness stop him from hiring an inept private investigator to find his mother, or being smitten with the town’s new hairdresser, Jo Wingfield (Heather Graham), after one glimpse through the salon’s window. Despite a bloody haircutting mishap, the two embark on an idyllic romance complete with puppies, sunny days in the park with Wingfield’s wheelchair-bound dad, Walter (Richard Jenkins), and picnics on the animal shelter rooftop where Noble used to lunch alone.

Like the classic screwball comedy, you never really believe for a moment that Noble and Wingfield sprang from the same loins. Since their six-month courtship is summed up in quick cuts, no foreshadowing plays on the sick possibility that they are blood relations: no townspeople remarking on their clean-cut physical resemblance; no noises Noble makes that could remind Wingfield of her father; no passionate recitals to one another from “Oedipus.” When they finally have sex, only the poster of Suzanne Somers on Noble’s bedroom wall seems out of whack.

The union comes to an abrupt end when Noble’s private investigator comes up with yet another maternal suspect, this time Valdine Wingfield (Sally Field). Jo heads back to Seattle and much to her mother’s avaricious relief back to ex-boyfriend and Seattle millionaire Jack Mitchelson (Eddie Cibrian). The bulk of the movie is Noble’s efforts to convince Wingfield that they’re not related trust me, no surprises are given away here.

If the taboo is the joke, revulsion has to be the punch line. Aside from the townsfolk making the occasional incest pun, “Say It Isn’t So” doesn’t veer too much from the average screwball comedy of errors and misunderstandings. Klein especially is made to be too sweet. The flawed leading men of the Farrelly brothers’ more critically successful “Kingpin” and “There’s Something About Mary” were victims of circumstances, but often circumstances of their own making. Klein, meanwhile, is just a nice guy trying to walk a straight line, while people are constantly running up and knocking him down and it ends up being not that exciting to watch.

The Farrelly trademark stock characters return, including the drooling invalid (Jenkins) and the trailer-trash older woman (Field). Pilot Dig McCaffey (Orlando Jones) is the double amputee with spare parts for every occasion who swoops in to help out Noble. Dogs and cats are largely spared the electrocution or drug overdoses of the brothers’ earlier films, but the sacrificial cow does make a comeback.

There are dumb and dumber moments to get some tittering going, as well as some gross-out numbers that will have people squirming in their seats, although obvious setups lessen the impact.

Maybe the Farrelly brothers have set too high a standard for their lowbrow comedy. Maybe Jerry Springer and his brethren have signed up too many subscribers to their taboo-of-the-month club to make even incest shocking anymore. Maybe we should breathe a sigh of relief and hope that the two boys don’t start looking for love and laughs in the morgue for their next flick.