Marriage is a commonly misunderstood word. It has nothing to do with paperwork or weddings, with titles or even commitments. Marriage is the process of fundamentally changing your definition of life.
It is the point where a person says "I am no longer going to live my life, I am going to live our lives". Where the union of the two becomes something more and distinct than the individuals involved. A friend of mine likens it to the Borg from Star Trek, calling herself "1 of 2".
But that is the essence of that union, a state of mind and heart where you can no longer make decisions for yourself. Where it is no longer your life, but two lives so intertwined they are one. And it has nothing to do with being co-dependent or any other pop-psychology term. For that concept inherently deals with you depending upon someone outside yourself. But in this marriage, there is only the union, no longer individuals, hence nothing outside of you. One friend defined it as the point where each is capable of make wise decisions for both, and each could place complete trust that those choices will be the best for both.
I think it is part of the dualistic nature of this world that such a marriage must be monogamous. Two halves can become a whole, but never three or more. Nor can you ever love or be or share with anyone as intensely as that "other part of you". For your attention can only be in one place at a time, and either it is on yourself, or on one other. And only when two have given their full minds and hearts to another, have reached that point beyond communicating or working or living together, when being part of that union is more important to both than being themselves, only then can it truly a marriage.
Perhaps the word Marriage itself should be replaced, relegating that term to the lesser bonds of family and commitment, of legal contracts and shared bank accounts. Perhaps there is no word in English for that union, just as there is no word for that moment of pure understanding and knowing that transcends labels and thoughts and opinions.
And perhaps there does not need to be a word for it. For as a great poet once said: