Not here its not - you can choose to marry a man or a woman.
The whole theory of dissolving an existing marriage to 'make things equal' makes a lot of assumptions about poly, the couple in question and how things 'should' be. Smacks a lot of the ONE TWUE WAY
and I think you'll find that there is no one TWUE way of practicing poly, or anything else.
My husband and I will never divorce simply to 'make things equal' - it would be unfair to him, to our children and to our selves. The fact that we choose to NOT remain monogamous has nothing to do with why we married in the first place.
Someone else has it noted on their signature - there are as many ways to practice polyamory as there is people practicing it.