OK, so here goes - my opinions, for what they are worth.
Some people see legal marriage as the highest form of commitment to a relationship. I see it as a legal document and nothing more. It has nothing to do with a degree of commitment or how much I love someone. I can be just as committed to my partner without a legal document saying so. If we have trust in our relationship and believe the promises that we made each other, then no government-sanctioned document is going to change that for the better or worse.
The other problem is that if legal marriage is seen as the highest form of commitment, and while the laws stay the way they are today (each person can only have one legal spouse), then by definition this is going to be ranking your poly relationships by setting one automatically above all the others.
So, for me, there are a large number of highly effective and desirable relationship options between having someone as a secondary and being legally married - it's more than possible to have multi-primaries, in the case of poly families living together. Even with secondary relationships, there can be a high level of commitment to the relationship without the sharing of the day-to-day running of a house.
I don't feel the need to have a formal ceremony to show family and friends how much I am committed to the ones I love in my life - they see that in the every day interactions. Commitments I make with my partners are between my partners, and I don't fell the need to share the making of those with my family or friends.
I'm not suggesting that anyone else should feel this way - just putting it out there as a different way of thinking than the standard that society tends to present us with.
"Listen, or your tongue will make you deaf." - Native American Proverb