And He [Jesus] 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."

Marriage is between man and a woman because God created humans as male or female, where gender (sex) is characterised by the genes of the person not by what they think: female having two X chromosomes (XX) and male an X and Y (XY). Any other form of marriage is considered a sin by God. Divorce is also a sin.