Marriage is a commitment to stay together for life. Family is the most compelling reason to make such a commitment, as children are naturally a lifelong relationship and long-term shared responsibility. Caring for the spouse is an extension of caring for the child.

Sure, people can get married without choosing to have children. There's just way less on the line. Divorce is much easier. Just split the assets and move on. No need to worry about custody issues, where the child will live, all the relatives that have a relationship with the child, etc.