In God's original design, human sexual conduct was to occur within the context of marriage between one man and one woman. The first chapter of the Bible says, “God created man in his own image, in the image of God he created him; male and female he created them” (Gen. 1:27). Differentiation of the human race into two complementary sexes (“male and female”) is the first fact mentioned in connection with being “in the image of God.” In Genesis 2, which describes in more detail the process summarized in 1:27, God said, “It is not good that the man should be alone; I will make him a helper fit for him” (Gen. 2:18). Genesis then applies the example of Adam and Eve to all marriages: “Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh” (Gen. 2:24). This “one flesh” sexual union was thus established as the pattern for marriage generally, and Jesus cites Genesis 1:27 and 2:24 as the normative pattern that God expects all marriages to follow (see Matt. 19:4–6). Furthermore Paul, as a good disciple of Jesus, likewise strongly echoes Genesis 1:27 and 2:24 in his two primary texts on homosexual practice, Romans 1:23–27 and 1 Corinthians 6:9. Jesus and Paul both assume the logic of sexual intercourse implied in Genesis: a sexual bond between a man and a woman requires two (and only two) different sexual halves (“a man” and “his wife”) being brought together into a sexual whole (“one flesh”).
This is further emphasized in the story of the creation of Eve from Adam's side:
And the rib that the Lord God had taken from the man he made into a woman and brought her to the man. Then the man said, “This at last is bone of my bones and flesh of my flesh; she shall be called Woman, because she was taken out of Man.” Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh (Gen. 2:22–24).
The word “therefore” connects the making of Eve from a part of Adam's body with the “one flesh” sexual union between a man and a woman in marriage: it is the reunion of the two constituent parts of a sexual whole. It is not another man who is the missing part or sexual complement of a man, but rather a woman. (Jesus emphasizes this connection between the two different sexes, “male and female,” in Matt. 19:4–6 and Mark 10:6–8.)