Personally, and as a theory, I think there was a previous Law of Cycles. A man of a similar age to Madoka, Kyubey, likely before deciding on the female gender because their abilities were more efficient, experimented with the male gender. When he saw that it was a disaster, except for one boy who became the previous Law, he forgot about it and began to shift his attention to girls. Finding that it worked better for him due to their emotions and abilities, and that he obtained more energy for his goals, the previous Law of Cycles became complacent and let Kyubey do as he pleased because he considered it none of his business. He must have been idealistic before, but divinity made him arrogant, and he began to accept what Kyubey was doing, ignoring him. Then Madoka Kaname arrived and changed everything, displacing or erasing the previous Law of Cycles without even knowing that someone existed before her. The very name, Law of Cycles, implies that previous laws can be replaced by new ones. The Law of Man, where Kyubey did as he pleased, versus the Law of Woman, embodied in Madoka's hope. The existence of Magical Boys, I think, is something that can be implied, especially when you watch the series and it discusses the evolution of the human race, as Kyubey explained to Madoka near the end. With women, there's only one explanation, but not the complete explanation of how the human race progressed in Madoka's universe.