Does loyalty exist in the wild? Does friendship exist in the wild? Does love exist in the wild? If there was no society, would marriage exist?
If there are only one man and a woman left on the planet, can they get married? Do they need a witness? Do they need a legal contract? Do they need a religious figure? If they can, we are bound by consistency to say that all religious and legal institutions are unneeded for a marriage to occur. The legal paperwork is futile, too.
Consider friendship. Almost everyone on this planet has a best friend. Are their religious or legal rules of friendship? How then are people so successful at having a best friend, some time, even a best friend forever. Do best friends forever (BFFs) make any vows to each other? How do they sustain their friendships? The answer is very simple-- people are naturally inclined to be friends. Just like a dog is naturally loyalty, humans implicitly understand friendship. Even children understand friendship. They don't have to be taught friendship.
People fail when they legislate friendship when they start to think of what to say, what not to say, how to act, and how not to act. In the west, political correctness runs wild, which often implies a set of behavioral rules which impede people's natural ability to connect and be friends. Friendship is predicated on freedom. There is no way one can reach that freedom via self-regulation. Say what you say. Be what you want to be, like little children. See how quickly you make friends. Thus, friendship is not a cause but a consequence or a recognition of some sort of implicit values being kept (e.g. truth, honesty, loyalty).
Similarly, marriage is a recognition of implicit values kept mutually and unintentionally. The vows of marriage externalize these values. The pronouncement of these vows means nothing if the implicit values are not mutually kept. When I say kept, I don't mean that the parties involved force themselves to keep them. If they are intentional about it, it implies that they don't have these these values. By the word recognition, I mean these implicit values were kept unintentionally. Marriage is a recognition or a summary of this relationship.
If the vows are kept explicitly but the underlying values are missing, it is fundamentally not a marriage, just like everyone you call a friend is not a friend (ever had a friendship where you and your friend did everything right, yet you couldn't feel secure in it?). A dog doesn't promise loyalty and you don't ask for it but you consider it loyal when he implicit values of loyalty are kept. Likewise, marriage is not a cause but a consequence or a recognition of implicit spontaneous love. This is what marriage means. It has nothing to do with religion and law, just like friendship has nothing to do with either. The marriage license is just as useless if the implicit values have been dropped. Laws, religions and political, only distract people from the truth.