Here's a nice summary:
1) The Theology of the Body is a rearticulation of the Christian Gospel rooted in terms of human sexuality, who we are as men and who we are as women.
2) In Genesis, we read that men and women are made in the image and likeness of God. Sounds nice, but who is God?
3) God is a communion of Love. God the Father pours himself out in love to God the Son, and God the Son receives that love and gives a total gift of Himself back to the Father in return by dying on the cross. And the love between them is so tangible that it becomes a third person, the Holy Spirit.
4) In the same way, in the sexual act a man pours himself out in love for a woman. The woman receives this love and gives a total gift of herself back to the man. And the love between them is so tangible that nine months later, you have to give it a name - it becomes a third person!
5) Therefore, the sexual act is meant to be nothing less than the number one sign and symbol that God has written into our very nature as men and women to be an image - a symbol - an icon - of who He is in His inner self - a communion of Love. That’s why Catholics save sex for marriage - because it is so good and so holy and so sacred. We say “no” to sex before marriage so we can say “yes” to sex in a much deeper way - in a way that most perfectly images the God himself.
There are some refinements needed here: the Holy Spirit is not a "child" of the Father and Son--point four would better begin, "in an analogous way..." But it does clarify the matter well.
From Intentional Disciples, via Amy Welborn's Open Book.