You keep asking the same question, I'll keep giving the same answer: because it throws thousands of years of human culture, tradition and social norms out the window.
Marriage has traditionally been between one man and one woman. This is true of all cultures throughout the course of human history. The concept of same-sex marriage is a recent phenomenon, over the past 30 years or so. It seems rather foolish to throw away thousands of years of culture, tradition and social norms over something that is, in the big picture, "brand new".