This sounds like part of Kurt Vonegurt Jr's Breakfast of Champions.
It has the exact scene you describe.
It's a novel, though it could easily be misremembered as a series of short stories because it's typically disconnected.
Here's the excerpt of the first time it's mentioned. I believe it comes back into the novel once or twice but I can't find where.
The full text of the novel is available online.
The movie theater where Trout sat with all his parcels in his lap
showed nothing but dirty movies. The music was soothing. Phantasms of
a young man and a young woman sucked harmlessly on one another's soft
apertures on the silver screen. And Trout made up a new novel while he
sat there. It was about an Earthling astronaut who arrived on a planet
where all the animal and plant life had been killed by pollution,
except for humanoids. The humanoids ate food made from petroleum and
coal.
They gave a feast for the astronaut, whose name was Don. The
food was terrible. The big topic of conversation was censorship. The
cities were blighted with motion picture theaters which showed nothing
but dirty movies. The humanoids wished they could put them out of
business somehow, but without interfering with free speech. They asked
Don if dirty movies were a problem on Earth, too, and Don said, "Yes."
They asked him if the movies were really dirty, and Don replied, "As
dirty as movies could get." This was a challenge to the humanoids, who
were sure then: dirty movies could beat anything on Earth. So
everybody piled into air-cushion vehicles, and they floated to a dirty
movie house downtown.
It was intermission time when they got there, so
Don had some time to think about what could possibly be dirtier than
what he had already seen on Earth. He became sexually excited even
before the house lights went down. The women in his party were all
twittery and squirmy.
So the theater went dark and the curtains
opened. At first there wasn't any picture. There were slurps and moans
from loudspeakers. Then the picture itself appeared. It was a high
quality film of a male humanoid eating what looked like a pear. The
camera zoomed in on his lips and tongue and teeth, which glistened
with saliva. He took his time about eating the pear. When the last of
it had disappeared into his slurpy mouth, the camera focussed on his
Adam's apple. His Adam's apple bobbed obscenely
