There are 2 things we are talking about here: Marriage and Civil Unions
Marriage is a religious institution that has been upheld by societies all over the world for thousands of years. Even if you look back into ancient times, when it looked like it was more political, it was actually more religious and tied to some kind of ceremony. It was always between a man and a woman. Even if a man had slaves or multiple wives, there was never a marriage between two men or two women. If we want to split hairs, God created Adam for Eve and Eve for Adam. There was no such thing as divorce or multiple spouses in the world before sin created that rift between God and man. Fast forward thousands of years and you get Jesus, who reminds us that it was out of the hardness of hearts of man that divorce and other things that tainted marriage happened. God did not make marriage for any other reason than for one man and one woman to become one flesh and to be fruitful and multiply.
What the State has the authority to do is acknowledge Civil Unions. Why? Because the State is not a religious entity. It’s just semantics that they claim to acknowledge marriage. It’s why, in the US, even if you are married in a church, you have to go to the courthouse to file for a marriage certificate. A Civil Union is what provides a couple things like tax breaks and other legal advantages. You can have a priest marry two people, but unless they go get it recognized by the State, they are not “married” in the eyes of the State.
Therefore, if people of the same sex want all of the economical and legal advantages of being married, give them a Civil Union. Who cares really? A Civil Union is only worth the ink and paper that makes it legal. It is not marriage. It is nothing the Church can recognize as a Sacrament that brings grace into the family. I may not agree with it morally, but legally, the State has no authority to “marry” people, so I am not afraid of “marriage” being redefined at all.
A Civil Union can be broken. A marriage cannot. Even an annulment is not a divorce because an annulment can only say that there was something in the beginning that prevented the marriage from ever being valid in the first place. Therefore the couple was never truly sacramentally married in the first place. The Church simply acknowledges this. It’s similar to when the Church proclaims someone a Saint. The Church does not make someone a Saint, God does. The Church just has the authority, given by Jesus Christ, to acknowledge the fact that God sanctifies a person in a special way after they leave this life.
Sorry for the long post, but it is important that we know the difference between Marriage and Civil Unions and who has the authority to administer/acknowledge each one.