Marriage is, first of all, a HUMAN institution. It is not, first, a state institution. It is not, first, a religious institution. Talking about the religious aspect of marriage is like talking about the religious aspect of a priest. You cannot have a "religious" marriage without a "natural", human marriage first any more than you can have a "religious" priest without having a "natural", human man first. The religious institutions elevate (in the eyes of the Church) the marriage to a sacrament but the religious institution does not create the marriage. The state regulates the marriage but the state does not create the marriage. The marriage would exist without the religion or the state. It is a natural institution that exists in all human cultures. It is a community that is created within a community.
Another important aspect of marriage is that the community recognizes it. Now there are and have been a myriad of human communities on earth and each community knows of marriage. The couple and their children make a small community and the larger community within which they live recognizes that. Even if they move to a new community their marriage will still be recognized as a marriage. Imagine, in the time of cavemen, a migrating couple moving from Asia across the land/ice bridge between Siberia and Alaska and meeting up with a new community in the New World. Most likely, the first thing the Native Americans would ask would be "Is this your woman?" and "Is this your husband." And if the answers are in the affirmative, the new community would probably (unless under the sway of a very corrupt leader who wanted to take the woman) respect the marriage. Even if there were such a corrupt leader, he would at least have to give some excuse for taking the woman and he would have probably have to kill her husband first before he could have her. Because every human community knows what marriage is.