As you’ve probably guessed from the TV commercials, or even just the billboards, “I Now Pronounce You Chuck & Larry” is a one-joke movie — and that joke might have seemed edgy back in 1977, when Billy Crystal was playing a gay man on “Soap.”

(Speaking of which, and in case you were wondering, yes, there is a soap-dropping reference in here.)

Crystal’s fellow “Saturday Night Live” alum Adam Sandler and likable lug Kevin James play Brooklyn firefighters who pretend they’re a couple to receive domestic partner benefits. This sets up a litany of obvious gags and adolescent one-liners, followed by a swift, politically correct embrace of gay culture.

Director Dennis Dugan (“Happy Gilmore,” “Big Daddy,” say no more) wants to have it both ways but never gets either right. It might seem offensive if it weren’t so lame.

Sandler’s Chuck Levine and James’ Larry Valentine try to convince the world they’re gay by picking up Wham! and Barry Manilow CDs and professing that they’re “riding the dude train.” They do this because Larry, a widower, is concerned that his kids won’t receive life insurance if he dies in a fire. (Among the flat subplots, Larry’s young daughter is a tomboy who loves baseball, while his son is practicing his tap routine to audition for “Pippin.”)

Suddenly, their fellow firefighters stop playing pick-up basketball with them and feel nervous about showering around them. And while a fraud inspector (Steve Buscemi in a weasly, unfunny role) starts snooping around suspiciously, their captain at the fire station (Dan Aykroyd) doesn’t want to know the truth.

So Chuck and Larry enlist the help of attorney Alex McDonough (Jessica Biel) to help them defend their case. Trouble is, Alex is astonishingly hot and Chuck, before turning into a faux-mo, was a ravenous ladies’ man. And so he agrees to be her buddy on shopping sprees and girlie wine nights, just to be close to her. Of course, because she trusts him so much, she lets him feel her breasts to prove that they’re real — we all do that with our gay pals! And of course she will eventually find out he’s straight and feel duped.

The most baffling part of all is that Alexander Payne and Jim Taylor, the Oscar-winning “Sideways” writers, are credited with having spruced up the script written a decade ago by Barry Fanaro (“Men in Black II,” “Kingpin”). You’d never know that anyone capable of creating rich, full characters — characters with hearts and brains — had anything to do with this.

Ving Rhames gets maybe two laughs as a tough-guy firefighter with a big secret, and Sandler pals David Spade and Rob Schneider show up in the obligatory uncredited cameos. Schneider’s actually is kind of offensive: He plays the wacky Asian minister in Niagara Falls who performs Chuck and Larry’s commitment ceremony, with shades of Mickey Rooney as the landlord in “Breakfast at Tiffany’s.”

But in the end, Sandler literally stands up in a courtroom to proclaim that homophobia is bad. “Chuck & Larry” could play on a Pedantic Cinema double feature with “Hairspray,” which teaches us that racism is bad.

The only useful lesson comes during the scene in which Biel strips down to a bra and panties and shows off her famously toned backside. The sight of her in all her muscular glory is enough to make you immediately flee the theater and head to the gym for some squats and lunges.

The added benefit of that, of course, is that you wouldn’t have to watch the rest of the movie.

“I Now Pronounce You Chuck & Larry,” a Universal Pictures release, is rated PG-13 for crude sexual content throughout, nudity, language and drug references. Running time: 115 minutes. One star out of four.

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Motion Picture Association of America rating definitions:

G — General audiences. All ages admitted.

PG — Parental guidance suggested. Some material may not be suitable for children.

PG-13 — Special parental guidance strongly suggested for children under 13. Some material may be inappropriate for young children.

R — Restricted. Under 17 requires accompanying parent or adult guardian.

NC-17 — No one under 17 admitted.