Poor Lassie can have no more puppies because it made someone think of sex. Rod Serling talked to Mike Wallace about censorship on TV in 1959. Not much has changed -you can still lose sponsors if someone complains. A commenter pointed out how Serling was a genius at getting around such censorship, and how it contributed to the art of television.
The irony here is that the format of "The Twilight Zone" was created as a way to get around censorship. If social commentary was disguised as tales about aliens and monsters then it was easier to get past execs and sponsors; we might not have "The Twilight Zone" if not for Serling having to find a way to work around them.(via Boing Boing)