I always figured it came from Bob & Carol & Ted & Alice https://en.wikipedia.org/wiki/Bob_%26_Carol_%26_Ted_%26_Alice https://en.wikipedia.org/wiki/Bob_%26_Caro

I always figured it came from Bob & Carol & Ted & Alice https://en.wikipedia.org/wiki/Bob_%26_Carol_%26_Ted_%26_Alice https://en.wikipedia.org/wiki/Bob_%26_Carol_%26_Ted_%26_Alic... and that by referencing Bob & Alice one got an extra frisson because of "Swapping partners, Bob and Alice kiss fervently, as do Ted and Carol..." - that is to say Bob (the free love liberal man) and Alice (the conservative commitment woman) are put together in all these anecdotes because it is naughty and naughtiness adds to the amusement. After a while of course nobody remembered the naughty movie and people just used Bob & Alice because those were the names one used for this kind of thing.