Many of my Christian friends are confused!
Why define marriage as between one man and one woman? Well, if it's not that, then what IS marriage? Between a man and a man? A woman and four other women? A person and himself? A person and their pet? Can we just say that marriage is whatever we want it to be?
Sure. But we can't say that and still be wise. The first man was given the first woman. That's God's idea about it all, and God is pretty smart.