What you are looking for does not have to be polygamy, depending on the spiritual commitment and relationship dynamics of everyone involved.
One question which comes to mind is if you would be open to a girlfriend who would also have the option of having another lover other than yourself?
Thats a tough question. I have already wrote and re-wrote several times only to have deleted it all. I guess the best answer is no. I say this because by saying yes, I feel like my wife is getting the short end of the stick, and I'm not looking for just an extra relationship on the side. I'm looking for a relationship with my wife and another woman. Except no three-way sex or anything.
I think I would be more accepting of the other woman having outside relationships, more so than I would be with my wife. (remember my wife likes the idea of me being with another woman, not visa versa). But the problem with saying yes, is that I feel like im diminishing the connection between another woman, and our relationship. It is not that I do not think she can be loving to two different people. But I feel that i'm looking for someone to come join our life!
As RP says, this is very complicated, because its easy to want this new person to conform to our lives, but we need to be equally supportive and conformative to her life as well. I am not sure this woman even exists, but hypothetically this is how I see it in my mind.