Here’s how bad “License to Wed” is: Even the outtakes at the end are lame.

It’s exactly what it looks like from the commercials — a one-joke movie, and that one joke isn’t even funny to begin with.

Robin Williams is constantly straining as Reverend Frank, the minister from hell who forces the newly engaged Ben (John Krasinski) and Sadie (Mandy Moore) to undergo a sadistic marriage prep course before they can say “I do.”

You’ll wish you’d said: “I don’t.”

As in the similarly desperate “The Ex” and “You, Me & Dupree,” Ben is the only one who recognizes that Reverend Frank’s methods are suspect; everyone else thinks he’s great. This conceit grows old, like, immediately.

It is refreshing, though, to see a new face in Krasinski as the straight man in one of these wacky romantic comedies. The co-star of TV’s “The Office” has unconventional good looks and an easy likability about him; more importantly, he’s not Ben Stiller or Luke Wilson or someone else you’ve seen a thousand times before.

But this is the second flat, shrill chick flick Moore has starred in this year, following “Because I Said So.” We know from movies like “American Dreamz” and “Saved!” that there’s more to her than her pretty, pop-star past would suggest. She just needs to choose roles that let her prove that. Meanwhile, Wanda Sykes, Bob Balaban and Grace Zabriskie are squandered here in bit parts.

So here’s the story, which bafflingly took four writers to concoct: Ben and Sadie meet awkwardly in line at Starbucks, fall in love and move in together. When he asks her to marry him — in front of everyone at her wealthy parents’ 30th wedding anniversary party, which is also awkward — she says yes, but tells him it’s been her lifelong dream to be wed at her family’s church.

That’s where Reverend Frank rules absolutely, with wannabe hipster slang tossed into his sermons and corny rhymes to teach the Ten Commandments (“Be chill, don’t kill”). And before he’ll marry any couple, he makes them pass his bizarrely intensive course, which he administers with his obnoxious, pint-sized choir boy sidekick (Josh Flitter, Shia LaBeouf’s precocious caddy in “The Greatest Game Ever Played”).

And so Ben and Sadie can’t have sex before the wedding, which Frank monitors by bugging their apartment, then eavesdropping on them in the back of a van with the choir boy in tow, which is just wrong. They have to argue with each other in an uncomfortable role-playing exercise. Ben is forced to try belly dancing as part of his group couples therapy — it’s just one contrived situation after another.

The only vaguely amusing example comes when they have to practice parenting with a couple of robot babies that look like mini Lex Luthors — but even then, director Ken Kwapis can’t help but go for the obvious gross-out joke by having one of the babies pee all over Ben and poop out a substance that looks like blue cake frosting.

Kwapis has also directed episodes of “The Office,” and he actually showed a surprisingly deft hand at potentially painful material with his adaptation of “The Sisterhood of the Traveling Pants.” Sitting through that movie again would be preferable, even for guys — at least the characters felt like real people.

“License to Wed,” a Warner Bros. Pictures release, is rated PG-13 for sexual humor and language. Running time: 86 minutes. One and a half stars 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.