The casting couch was never on tv. I think it was pornhub or something like that.
The joke was the juxtaposition between innocent family friendly couches that are fairly universally known and one that was raunchy but just as well known.
Using juxtaposition as an element of humor is a well studies form of humor, check out "incongruous juxtaposition theory" which is a theory for what makes things humorous. I know you have said you have autism so perhaps understanding the formal theories of humor might help you understand why jokes like this are funny? I hope that helps.
https://en.wikipedia.org/wiki/Theories_of_humor#Incongruity_theory