Eventually, when a father/mother relationship breaks down non-amicably, force must be used to enforce custody, the division of property, etc.
Eventually a legal definition will arise regarding mother/father relationships that will amount to marriage in everything but name.
Moreover, the duty of the State is the preservation and promotion of the Common Good, as classically understood and mentioned in the Preamble to the Constitution. The State has a natural right to preserve and promote marriage, since families constitute the fundamental building block of society. All citizens pass through the institution of the family.
The alternative is the Soviet ideal; free love and the annihilation of the family. Our society is rapidly moving in that direction.
“Eventually, when a father/mother relationship breaks down non-amicably, force must be used to enforce custody, the division of property, etc.”
A simple binding legal contract could accomplish that, and skip the state “marriage” all together.