This specifically refers to a biological phenomenon in animals where males will have sex more often if they are presented with a variety of partners, rather than the same partner over and over again.
The source is humorous:
The President and Mrs. Coolidge were being shown [separately] around an experimental government farm
When [Mrs. Coolidge] came to the chicken yard she noticed that a rooster was mating very frequently. She asked the attendant how often that happened and was told, “Dozens of times each day.” Mrs. Coolidge said, “Tell that to the President when he comes by.”
Upon being told, the President asked, “Same hen every time?” The reply was, “Oh, no, Mr. President, a different hen every time.” President: “Tell that to Mrs. Coolidge.”
This can be extrapolated beyond sex to life in general. Humans get bored with the same thing over and over again, and will exhibit behaviors more enthusiastically in response to variety and novelty.