Why did ancient people get married first and then?

In fact, there is a certain basis for this, because many ancient books have recorded the saying about eight-character fortune telling, so many people, especially those in rural areas, are used to counting the birthdays of both parties first when they get married. If the birth dates of two people can match each other, it will not only make their lives happy, but also benefit future generations. Just like I heard a story about Yuan Shikai told by the older generation in the village: When Yuan Shikai was seventeen years old, I heard her fortune teller say that Miss Yu was good. Yuan Shikai's two uncles studied it for a long time and thought that her and Yuan Shikai's birthdays were a good match. If she can marry into this family, it will promote the geomantic omen of Yuan family.