Romantic comedies always rely on some far-fetched scheme, so perhaps “Zack and Miri Make a Porno” is simply a 21st-century example of a solid tradition. But the title, of course, gives away the game.

In this new comedy from “Clerks” scribe Kevin Smith, the stakes are the same as ever: true love, expressed by two people who really do love each other but just don’t realize it yet. The update, in this case, involves the pornography business.

Zack (Seth Rogen) and Miri (Elizabeth Banks) are longtime platonic pals and housemates. The economic climate has gotten so bad that their only recourse is to shoot a porno movie, featuring themselves and a few enthusiastic recruits.

I’m sure there have been stranger pretexts for screwball comedies, although offhand I can’t think of any. “Zack and Miri” is based on Kevin Smith’s customary brand of yin-yang screenwriting: take the filthiest, most outrageous subject matter and make sure it’s in the service of a sweet-natured purpose.

Even though Smith is exploring every body-part joke known to man, he is also at his corniest in “Zack and Miri”; he’s a foul-mouthed provocateur with a soft center. He’s also a screenwriter with a rare sense of rock-solid structure.

But “Zack and Miri,” despite some belly laughs, doesn’t quite hold together the way some of Smith’s previous goofs have. Maybe this time the subject matter brings up too many uncomfortable “Now wait a minute, would that really happen?” moments.

Seth Rogen, whose status as a Judd Apatow leading man (“Knocked Up,” “Pineapple Express”) has turned him into the Cary Grant of arrested-adolescent movies, segues neatly into the Smith universe, and Banks is a game leading lady.

Smith also has a few of his regulars around: Jeff Anderson from the “Clerks” movies, Jason Mewes from everything. Craig Robinson, a regular on “The Office,” has some funny bits, and real-life porn veterans Traci Lords and Katie Morgan, add, I guess, verisimilitude.

I usually enjoy the profane soliloquies Smith writes for his characters, and the way he embarrasses them (and the audience) in order to lead them to a new kind of thinking. But this time out I can’t quite buy the world he offers up, the good cheer of making a porn movie among friends and strangers.

Having people fall in love while shooting a porno is a zany idea in the abstract, but the yuck factor here is a little too high to completely sell it in the flesh, as it were.