I have been getting lots of notices from various organizations about the Netherlands accepting the polygamous marriage of immigrants (typically Muslim). The notices have been written in a horrified tone similar to homosexual "marriage". While I do not think that polygamy is the way God designed marriage, I do consider it a marriage. Why? Because God did. Throughout the Old Testament there are polygamous marriage. God never has people send away their second or third wives. He did have them send away their non-Jewish wives during the rebuilding of Jerusalem. God never sends a prophet to confront a leader with many wives. He did send Nathan to confront David and (in the New Testament) John the Baptist to confront Herod about adultery.
Now I agree that God planned for marriage to be between one man and one woman for life. That is what he created in the Garden of Eden.
That fact doesn't stop us from recognizing remarried couples as married. We recognize them as married whether or not they got divorced for scriptural reasons (adultery would be a scriptural reason, he golfs too much would not).
My mother has told me of a dilemma in the African church. Many people when they come to Christ are in a polygamous marriage. She said most of the time they must send away their extra wives and kids or leave their husbands before they can be baptized. I think this is a grave error. Those kids need both parents. I would baptize them and just admonish the men not to marry any more women. I would admonish the women to stay married even if their unbelieving husband marries again.
I know it isn't ideal, but to me it seems more in line with what God would do (and has done in the past).