For me, the word marriage is not "charged" with ancient traditions where the wife belongs to the husband. For years, I was anti-marriage and referred to it as a "license to fuck." I felt it was just a way for people to get away with having sex without being ostracized for it. I never cared much for the idea of marriage and have always hated that married people get a better tax break than singles, and so on.
While I did grow up dreaming of someday finding someone to "take care of" me and live with in a nice house, it still took me by surprise when I reached the point where I did meet someone I wanted very much to marry, and we did. I was ecstatic to get married, and we never once thought about all those historical connotations many have attached to the concept of marriage. We simply wanted to be together and wished to commit to each other in a way that our family and friends, and the government, recognized. Now I am getting divorced. I am the fifth generation of women in my family who have been divorced/abandoned/separated, so I had also observed the fact that relationships aren't guaranteed to last forever.
So, I kinda see it from both sides. And when gay marriage became an issue, I have stated many times that I think the states should be in charge of, and grant, civil unions for everyone, no matter whether they are gay or straight or whatever, and let marriages be the domain of religious institutions. Then the legal end is satisfied, and those who feel a marriage is important can go and do it as an extra step in a church or temple or whatever, and it is up to the religious institution whether or not they will marry someone. The state can do the legal paper thing, the religious institutions the marriage thing. In other words, instead of gays aspiring to get married, why not have everyone get down on a level playing field and let straights all start with civil unions. Now, as far as poly's, I know some create LLCs instead of civil unions.