It is more about intent. Being married is an explicit commitment to one another for life (nominally). That provides a foundation where a bad day, month, or year

It is more about intent. Being married is an explicit commitment to one another for life (nominally). That provides a foundation where a bad day, month, or year is okay because you are committed, and that is a place of security for children who know their parents are similarly committed to them. Cohabitating couples are less likely to stay together even if they eventually marry. There's something about commitment to marriage that leads to better outcomes in relationships. The feeling of permanence for the future improves the experience of the now.