---let us look at the expression of sexuality in marriage [taken from the article by Fr. Paul S. Voisin, C.R: 6th commandment "You shall not commit adultery", The Bread of Life, Vol. 28 (5, Nov./Dec.2004): 23]
This is the plan of God we discover in Genesis, and throughout human history. This vocation is a response to the call of God to intimacy (unitive) and to the bringing of children (procreative) into the world. This sexual union is a sign of that spiritual communion they share, sanctified by the Sacrament of Marriage. In Familiaris Consortio #11, the Church proclaims: "Sexuality, by means of which man and woman give themselves to one another through the acts which are proper and exclusive to spouses, is not something simply biological, but concerns the innermost being of the human person as such." This conjugal fidelity within the marriage covenant becomes a sign of the faithfulness of God in the covenant with us. And, as we have seen in the Ten Commandments - God keeps the covenant!