The 20th Amendment.

The year is 1950. Food comes in the form of little pills - such as a pill for eggs and another pill for bacon. Coffee comes in the form of a spritz from a spray bottle, administered by a maid. One day Haley opens the newspaper to discover that a 20th amendment has been passed, "compelling every male citizen to have as many wives as he can support...because too many unmarried men are leaving this country due to the enforcement of Prohibition, and in order to protect the American home and the country's future population." So Haley immediately opens his little black book and before you know it has gathered a whole harem, one wife for each day of the week.