"Marriage" -- the word means different things to different people, just as the word "poly" means different things to different people. I've always "believed" in marriage, even as I've always "been" poly in some sense. During my first serious relationship, which led to an engagement, I knew that monogamy was not my ideal. That was twenty-seven years ago.
I didn't object to monogamy from a sense that life-long commitment couldn't work. I simply thought that a single commitment was too narrow -- though some would say that even one life-long commitment is too much to hope for.
My ideal of marriage was built upon the premise of a very special kind of friendship. My ideal friendship is something like the love of Odysseus and his crew. They have many adventures together. They are committed to one another. But they also have separate lives. They may have adventures on their own. But they never lose their connection to one another. They never lose their love and respect for one another. And, in some important sense, they never cease their commitment to each other.
Recently, someone strongly defended "committed" relationships to me. I asked her what she meant by "commitment". We left the question unanswered. Now I would say: "A commitment to each other's integrity". For me, that is the most important. Second after that (for me) comes the sharing of physical things: sex, finances, property, children.
I also never considered "marriage" to be something that could be conferred on me by the words of clergy or government official. It was for me something that others might recognize or acknowledge -- but ultimately it grew from the hearts of the individuals themselves.
I never thought of marriage as a vehicle for raising children, or a reason for special privileges
I never thought of marriage as something that should be forcibly maintained against the wishes of the participants.
These have been my ground rules. Nevertheless I do "believe in" long-term "committed" love-relationships. In some kind of "spiritual" sense, I do "believe" in marriage. But is the marriage I've described above "marriage" at all? Might not what I've described include "friendship" and "family relations" as well? I need to think about that some more (and consider whether or not it matters).