Marriage is instituted and formed by God as a sacred and mutual bond between a man and a woman creating a one flesh relationship.
Everything else about marriage hangs on this core definition within God’s structure and creative order.
Genesis 1:27 So God created man in his own image, in the image of God he created him; male and female he created them. Genesis 2:24 Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh.