Biblical marriage requires a one-flesh union, but not every one-flesh union is a marriage.
After uniting for life the first man and woman in history, the Holy Spirit editorializes in Genesis 2, “Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh.” The Lord Jesus affirms this statement in Matthew 19. It is not mere patriarchal opinion.
Now notice what he did not say. Sometimes that is as important as what is said.