For me, having come into non-monogamy after marriage, I always looked at it as something we shared.
Marriage for me is about tradition. I believe in the tradition of marriage, I wanted the white dress, and I took my husbands name after marriage. I will never marry again. He is the only one to ever be my "Husband"
We also have a d/s relationship. My marriage vows included, "love, honour and obey" (I have to be reminded about the obey part)
I wanted my children to have their father's name.
My capacity to love another or have a non-monogamous relationship is not exclusive of that. It is inclusive. I want/need family. Some is my blood family. The rest is my chosen family. My husband and family comes first in my life - and when you are a part of my chosen family, you are a part of my family for life.
I have read Redpepper's description of having a "tribe" I can see how that description fits. And I love reading her blog
So why marry? Because I wanted to. And he wanted to. And no matter what, he's my chosen family.