Reproduction of course

In the end everything is made for the purpose of reproduction. 

Sexual attractions, even sexes themselves, are made for reproduction, for plants and animals alike.

Families make the best social structure for raising up children. The partnership of a couple makes it easier and efficient to

look after children so that they can take turns babysitting, take turns making schedules for children's doctors' appointments and take turns on doing many things. And they have better resources putting income, time and labor together.

The center of marriage is children, babies, tots and school age children.

When children grow up, marriage lost its meaning. Many couples decide to divorce at this stage. Some start looking after grandchildren, just like they did before to their own kids !  Americans do that too if live nearby. They should have divorced if not for the next generations.