Some Pharisees approached him, and to test him they said, "Is it against the Law for a man to divorce his wife on any pretext whatever?" He answered, "Have you not read that the creator from the beginning made them male and female and that he said: This is why a man must leave his father and mother, and cling to his wife, and the two become one flesh? They are no long two, therefore, but one body. So then, what God has united, man must not divide.
They said to him, "Then why did Moses command that a write of dismissal should be given in cases of divorce?" "It is because of the hardness of your hearts," he said, "that Moses allowed you to divorce your wives, but it was not like this from the beginning..."
Gospel of Matthew 19:3-8 (emphasis mine)
John Paul deliberately begins his study of man (anthropology) by refering to Christ. By doing so, JP II is pointing the full revelation of man in Christ and that Christ is the authority to first be refered to. By refering to Christ, we now see that He points us to the beginning. This beginning can only mean one thing, the story of Creation.
By Christ pointing us to the beginning, He is showing us how the break from sin is something that involves all of history. Christ is not our only focus, but so is Original Man. By looking at Original Man, we can see the roots of our humanity, of our desires, of the meaning of what it means to be a human person. Thus, the redemption of human sexuality is not something that is simply for a certain time in human history, but it involves all of human history. All this history of humanity is thus brought to us through Christ, who is the Alpha and the Omega. Christ brings all history to Himself.
Christ is thus calling us through the discussion with the Pharisees to go back to the beginning to see our original calling to be one flesh. Christ is showing that this distortion in sexual relations is not something that is inharent in the nature of the human person, but instead, it is a result of sin.
The goal for JP II with Original Man is to show what the status quo was for Original Man, what it is that is still an echo in the human person today. Sin has distorted the perfect communion man had through the one flesh union of male and female. JP II takes our current state and flips it, to see, though not fully (he admits full well that this cannot be fully understood, that it is still a mystery) the mystery of why God created us to be male and female. The answer will astound you, as the reasoning behind it goes to the very core of what it means to be a human being!
Christ's mission is to restore that order through His death on the Cross. Because of Christ's death on the Cross, we are now able to re-orientate ourselves through God's grace towards that good that is experienced by each human person. Christ enables us to re-orientate ourselves back towards God, to the way it was in the beginning. This journey will be one of prayer and insight as we go to the depths of human sexuality, of the human person.
* = When I speak of man in bold, I am talking about man in general, not man as male. That is, man as the human person, fulfilled through male and female.