We call this the "sacramental principle" because we become aware of it when we consider the sacraments. For example, how Jesus took bread and wine and told us that when we bless them and share them in remembrance of him they are his body and blood — and he assures us that whoever eats his body and drinks his blood has eternal life. Or how we enter the kingdom of God when we have the birth from above by water and the Holy Spirit which makes us God's adopted children. In other words, God uses matter, the things of this world to communicate spiritual realities to us. At Christmas we are reminded of another instance of that principle: the Incarnation. John's gospel tells us that the Word of God — through whom the universe was created, and who is himself God — became flesh, to save us from evil and bring us God's life and love. God did not work simply in a spiritual way. He chose, typically, to use the matter of the world he had created — human flesh, in the person of Jesus — to accomplish his purpose.
That is why for us, sex ought to be more than a physical action. It is not just about the pleasure it gives. To us it is a representation of what God is all about. It should represent God's interior life as well as his relationship with those he loves. Sex tells us something about God, speaks to us about God. The man and woman were created in the image and likeness of God, it says, and we take that to imply that it is in the context of a relationship like theirs that it is possible for sex to deliver the message about God fully and completely. One great thing about sex is that this sacramental principle is part of it from creation itself, so that it can teach us about the divine even if we don't believe in the Judeo-Christian faith.
God's love is unconditional, unending, unbreakable, and creative (including creative of life). For sexual activity to tell of God's love according to the sacramental principle, it needs to possess the same characteristics, to be in he context of a relationship of that sort. When "the two become one flesh" in such a relationship, it is like God becoming one with us; it is like the unity of Father, Son, and Holy Spirit; and it is like being with God in heaven.
