According to the Catholic Church, marriage is a sacrament created by God himself to allow the physical and spiritual union of one man and one woman for the sacred purpose of procreating and raising children. There is no other reason to be married and, therefore, no other form of a partnership shall be recognized as a marriage. The state has allowances that grant them the ability to call certain unions a marriage in the eyes of the law. Despite this Earthly fact, a marriage in the eyes of the law does not prove it's legitimacy. Only a marriage recognized by God is a true marriage. So the debate between whether or not a marriage between same-sex couples is irrelevant. God does not recognize the marriage, therefore it is not so.
The state has always been involved in marriage. As far back as ancient Rome, marriage was a legal matter under the remit of imperial law. It is an entirely legal construct. Socially we do not need to publicly declare our commitment to another person. It is implied in our decision to spend all our time with them, share responsibilities, and share our living quarters.