We can't go back to primitive society, and even if we do, we can't understand what the ancients said, so I can only use a guessing method to face this problem.
In anthropology, there is a way to study the situation of primitive societies by studying some old traditions handed down from the past or some uncivilized peoples, so let's speculate on the origin of the cult of reproduction from these points.
First, the idea of "many children and many blessings"
This is not to say that primitive societies had many children and many blessings, but that we adopt some traditions that have been passed down from generation to generation.
For example, in the very primitive society, the use of public ownership, one family and one son, at this time every child of each family has the same productivity, and then the materials obtained are also similar, then the public ownership system has the possibility of existence.
But over time, you have three children, my family has five children, and his family has one child. In primitive society, when the fishing, hunting and gathering economy was dominant, it was natural that the family with five children was the most productive. For example, if you have five children in the family, you may be able to hunt large animals; if you have only one child in the family, you can only hunt small animals.
Therefore, in this case, inequality has arisen, and the natural status of the large number of people in the family is higher, and the natural status of the small number of people in the family is lower.
Then at this time, if an armed struggle breaks out, it is natural that the family has the advantage of more people. Therefore, in primitive society, the more people there are, the greater the power.

Second, the mortality rate of primitive society is extremely high
We now understand that women need a clean and sterile environment for childbirth, and when necessary, surgery such as caesarean section is used to help with childbirth.
Further on, we watched the TV series, in ancient times, childbirth required at least a few clean cloths, some wine, a basin of hot water, etc. These are designed to help women maintain a clean environment.
We continue to go back to the primitive society, naturally not so exquisite. Whether women have heavy bleeding, difficult labor or germ infection during childbirth, women in primitive society can only rely on their own bodies to carry hard.
In addition, the health of the children born is also difficult to be guaranteed, and the mortality rate is extremely high. Faced with this situation, only as many children as possible can be born, and then the population can be barely increased by means of high birth rates. This has formed a pattern of population growth with high birth rate, high mortality rate, and low population growth rate.
Therefore, the ancient people practiced fertility worship in a way that also had a reason to increase the population growth rate by increasing the birth rate.
Third, awaken man's primitive desires
This is purely personal speculation. I watched some TV dramas before, in which before young people get married, the man will often be sent a "spring palace map" by his father, and the woman will often be "nodded".
So I wonder, is there a possibility in this fertility cult that awakens man's primitive desires? Through some ritualistic rituals, some demagogic artifacts to awaken man's primitive desires.
But to put it bluntly, this awakening is also to increase the birth rate of the population, and thus obtain more labor and food to ensure the reproduction of the tribe.
Text: JAL Tongjian Map: From the Network and The Site, Invasion and Deletion