Plot hole: A block of cement that big would be very heavy, far too heavy for Lucy and Ethel to carry, not to mention someone would have noticed them carrying back to their hotel room.
Revealing mistake: Lucy, Fred and Ethel are admiring the celebrity footprints in cement outside Grauman's Chinese Theatre in Hollywood. In several scenes, the shadows cast by Lucy, Fred, Ethel and a passerby make it very obvious that what appears to be the ticket office, entrance and front wall are actually just a painted backdrop.
Answer: According to Snopes.com, there is no definitive answer, but the mid-1960s is the most verifiable date with "The Munsters" being cited as the first, although others claim "The Brady Bunch" showed the first couple seen in a double bed. An early TV show from the late 1940s titled, "Mary Kay and Johnny" is also thought to have shown the married couple's bedroom as having a double bed, although probably not with them in it. However, this was when TV was aired live, and there are no surviving episodes, only anecdotal accounts.
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Something that is funny is that in the movie "A Christmas Story," they show the parents having two twin beds in their bedroom. In a real situation, they should have shown them having a double bed. Lucy and Ricky had twin beds pushed together in an early episode, which would have been pushing television boundaries in that time.