Marriage between a man and a woman was instituted by God with Adam and Eve. Genesis 2:24 states: "Therefore a man leaves his father and his mother and cleaves to his wife, and they become one flesh."
In Matthew 19:4-5, Jesus reaffirms this: "He answered, ‘Have you not read that he who made them from the beginning made them male and female,’ and said, ‘For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one’?"
I doubt that this will satisfy the culture worshipers. We may have to look at biology or chromosomes.