Per OP's revision, it's from Family Guy, Series 9 Episode 9, titled "And I'm
Joyce Kinney" where Lois makes a porno and shows it in church. The word porno
in the sense of a single instance of porn (i.e. - a video, in this case) occurs
repeatedly in the episoide - for example, Brian says "Lois! You were in a
porno?".
 
Seth Macfarlane (creator of both Family Guy and American Dad) has an excellent
ear for language usage, and I have the distinct impression he likes to raise
little quirks like this. Probably with all the success he's having, he actually
has people around him actively seeking out such tidbits for him to consider
using in his cartoons.
 
Digressing slightly, I think there's a reason why "quirks of English language"
occur more often on "adult cartoons" than we might otherwise expect. Native
speakers subconsciously "know" much more about usage than they're normally
consciously aware of. When you see something like OP's example, there's a
strong tendency to think "Yeah! I know that too! I'm smarter than I thought!".
You end up feeling good about yourself when you watch it, so you're up for more
of the same.
 
Regarding the specifics of "accepted usage", Lois is correct that "a porn" is,
well, weird, - "porn" is an uncountable mass noun ("The Church disapproves of
porn"), or an adjective ("Lois made a porn video"). Sometimes "porno" is used
as an adjective too, but never a mass noun - usually, it's a countable noun
("Brian made several pornos", each a single item of pornography). A porno is
normally video - I've never heard it used of pictures, writing, audio
recordings, etc.