And this is the encyclical reference I use for believing that the state needs to be out of the marriage business. Now.
Arcanum is brilliant, much more unknown than Humanae Vitae but just as prescient and important, in my opinion. He lays out the danger of conditioning the populace to believe marriage is whatever the civil authorities say it is at any one time. Eventually many will simply accept any possibility, as long as the state accepts it first.
FReegards