Men are less sexually attracted to the same female partner over time and seek out novelty. During our evolution, it worked as a way to more effectively spread our genes.
The term "Coolidge effect" was first suggested by behavioral endocrinologist Frank. A. Beach in 1958. He attributed the neologism to:
... an old joke about Calvin Coolidge when he was President ... 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."