From Focus on the Family's talking points on Marriage:

- Marriage provides important benefits to society that no other institution can. Marriage:
- Regulates sexuality. Marriage establishes sexual guardrails, which remain a requirement for all successful societies.
- Socializes men. A society's most serious problem is the unattached male, and marriage links men to women who help channel male sexuality and aggression in socially productive ways. Marriage and parenthood socialize men to care for and respect their wives, other women and children.
- Protects women. Without a social norm of monogamy, women become commodities to be used and discarded.
- Provides children with a mom and a dad. Both mothers and fathers contribute in unique and irreplaceable ways to their children that prepare them to contribute to, and continue a flourishing society.

Read the rest here.