I see my spouse as the no-contest first-place winner in my list of important people in the world. The math doesn't work out for this to be mutual in more than two people.
Many people around the world have a different experience. I'd encourage you to research other viewpoints before making a blanket statement that a certain relationship style "doesn't work out."
"I see my spouse as the person of the opposite sex that I want to marry."
There are real challenges to handling society with n > 2 marriages (pension splitting, wills, children, etc), but how a person feels about their personal choices shouldn't impact consensual adults that feel otherwise.
The book "Opening Up" by Tristan Taormino is a great starting point. (http://www.amazon.com/Opening-Up-Creating-Sustaining-Relatio...)