In Genesis we read:
And the Lord God said, "It is not good for the man to be alone. I will make a helper suitable for him." . . . So the Lord God caused the man to fall into a deep sleep; and while he was sleeping, He took one of the man's ribs and closed up the place with flesh. Then the Lord God made a woman from the rib (side) he had taken out of the man, and he brought her to the man. (Gen 2:18, 21 22)
Why did God do it that way? Why create one being and then take a part of that being and create a second, differentiated yet complementary being who is "bone of his bones and flesh of his flesh," a being who is sexually, emotionally and in other ways different, yet of his own substance? Upon seeing her, Adam could have observed, "It's me . . . but not me." Well, if you think about it, it does sound like the kind of thing you might expect a Trinity to do.
The Trinity (Father, Son, Holy Spirit) is a family, and thus man in God's image must be made a family as well. Therefore, a man cannot completely realize the essence of his existence until he learns to exist with someone and for someone. Both relationship and communion are crucial to this process.
And so we see from Genesis 1 and 2 that God created woman from the side of man so that the man would not be alone. From the teaching of the New Testament, saints have since discovered that He also created the Church from the side of the second Adam—Christ—for the same reason—for intimate fellowship.
Back in the Genesis account, we note that the newly created Eve was Adam — his very flesh and bone, and for that reason, the Bible says, Adam called her woman, and, for that reason a man is to leave mother and father and be united to his wife to become one flesh (v24).
For what reason is man to marry a wife? Because woman was originally a constituent part of man, she must return to become one with him again, so that the full expression and design of God's image in human beings can be revealed.
Here we have another parallel between the Old Testament type and the New Testament fulfillment. Eve was to reunite with her source and become one with him—just as we are with Christ, as He prayed in John 17. Sexuality, therefore, is a prefigurement of the intimate relationship that God desires to have with man. In fact, the marital union and covenant, in all its dimensions, is meant to gloriously reveal the very image of God in ways that we can only begin to understand.
Ah, but there's more to this mystery than can be seen on the surface. The union of a man and a woman in Holy Matrimony is not literally the permanent recombining of two bodies into one. This is mystery that reaches depths of meaning beyond what our present intellectual capacity can grasp.
Clearly however, what woman is as a part of man is not tied to individuated pieces of flesh and bone, but is far broader and more profound than that. She is the necessary compliment to him that together reveals the glory of the image of God in humanity. Her parts and his parts each have their own order and function. Together and rightly ordered, their united differences ignite the power and glory of creation itself, which is the consummate activity of God from the beginning.
So God does a two stage creation of man. First he makes the full orbed being (Adam, which in the Hebrew means, mankind). Then in phase two, God removes woman from Adam's side and makes Eve a separate being, though of Adam's substance, designed to ultimately reunite to her source through the mystery of Holy Matrimony.
And the spark, the power of that union is meant to gloriously reveal the very image of God to angels and archangels and all the company of heaven and earth. That is why Satan fights tooth and nail to pervert and distort rightly ordered human sexuality, holy matrimony, the family, and fatherhood in particular.
In fact, the amount of time and effort that Satan expends to destroy the image of God reflected in marriage, fatherhood and human sexuality is a barometer of just how incredibly important it is to God's plan and the expression of His glory.
Beloved, there is a profound and awesome reason for the way God ordered the creation of man—one that is commented on throughout Scripture, and one that we must observe if we are to find the fulfillment of our very being as humans. It is ordered as the union of a man and a woman in marriage—heterosexual and monogamous—an order that Jesus unambiguously reaffirmed in Matthew.