What you are looking for does not have to be "polygamy," depending on the spiritual commitment and relationship dynamics of everyone involved.
Would you be open to a girlfriend who would also have the option of having another lover, other than yourself?
That's a tough question! I have already written and rewritten responses several times, only to delete them all. I guess the best answer is, honestly, no. If I said yes, I'd feel like my wife were 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 vice versa.) But the problem with saying yes is that I'd feel like I was diminishing the connection between another woman and our relationship. It's not that I wouldn't think she could love 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 it's easy to want this new person to conform to our lives, but we need to be equally supportive and conform to her life, as well. I am not sure this woman even exists, but hypothetically this is how I see it, in my mind.