Again, we may flounder helplessly without a common definition of "marriage". If I knew your definition, I might be able to agree.
Back at you.
If marriage is mainly & merely a civil contract, why give it special treatment different from other relationships?
Civil marriage is a contract. Our nation and others give it special treatment because they have found out over time that it is more efficient for the served society and its individuals to do so and beneficial for the served society and its individuals to do so. For example, soceity benefits when people make a life long commitment to love and care for another specific person, so society, via the government that serves the society, grants such relationships some special treatment as a means to encourage such relationships. Historically marriages have served addtional purposes and provided other benefits (see, for example, creating a connection between competing families, tribes, or even nations, providing for a desired transfer of wealth, providing for more than one parent for a child).
A reason to disagree would be James Rix' analysis that marriage is a social construct devised within each society as it sees fit & will not necessarily be consistent from one society to another or from generation to another.
Civil marriage is a social construct. Unfortunately, societies don't always come to the best social constructs for themselves. Currently, with respect to plural civil marriages, the society of the entire U.S. has failed itself, and with respect to SSM much of the society of the U.S. continues to fail itself. At least in my view.
So, your view of marriage is True for you & mine is True for me but there is no marriages apart from our imagination. It recalls a line from the 1st Matrix movie: "There is no spoon!"
No, civil marriage is a real thing, not just something in our minds. Currently, civil marriage in the U.S. isn't what I think it should be, but that doesn't make civil marriage a figment of my imagination.
I've got lots of views for which I can't see any reason for anyone not to agree. I don't imagine my lack of imagination makes me right.
Let me add that I see many reasons to agree that plural marriages and same sex marriages should be allowed.
I don't think that one's lack of imagination makes one right, but I do think a lack of imagination can make one wrong.