What is marriage to start with? It cannot be what it is today -- a man-made ceremony.
"Yet you say, "For what reason?" Because the LORD has been witness between you and the wife of your youth, with whom you have dealt treacherously; yet she is your companion and your wife by covenant. But did He not make them one, having a remnant of the Spirit? And why one? He seeks godly offspring. Therefore take heed to your spirit, and let none deal treacherously with the wife of his youth" (Malachi 2:14-15).
Thus by the Lord's statement in Malachi, we learn that it is not the ceremony that creates a marriage, but the covenant established between a man and a woman. (See "What are covenants?" for more information.) Much of our traditional wedding ceremonies revolve around the establishment of a covenant. The exchanging of the vows is a declaration of the stipulations of the covenant. The rings are the witness to the covenant. The reception dinner echoes the fellowship meal between agreeing parties. But most of all, a covenant is a vow before God.
Thus King Lemuel records that his mother spoke to him as, "What, my son? And what, son of my womb? And what, son of my vows?" (Proverbs 31:2). Lemuel was her son, the product of her womb, the result of her wedding vows.
"And He answered and said to them, "Have you not read that He who made them at 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 flesh'? So then, they are no longer two but one flesh. Therefore what God has joined together, let not man separate."" (Matthew 19:4-6). Even though we regulate marriages through civil laws and civil ceremonies, the actual marriage bond is controlled by the Almighty. It doesn't matter whether those marrying are Christians or not (see I Corinthians 7:12-14), God regulates marriage and people will be judged by their obedience to His laws.
Thus the idea that man can alter the definition of marriage is laughable. God established it as a covenant union between a man and a woman. The definition is fixed and unalterable.