God created humanity to love - that is our fundamental vocation. The human body expresses this call. Christian Revelation recognises two ways in which man can fully express this vocation: marriage and celibacy.
Our sexuality then concerns the very core of our person - who we are called to be, and is not something which is a purely biological function.
Physical self-giving is a lie if it is not a sign of total self-giving. Married love includes the fertility of the spouses. Marriage is the only place in which the total self-gift of the person can take place. This community of life and love comes about through the free consent of a man and a woman. Marriage does not come from society or from culture, rather, society and culture affirm the consent which arises from the interior of a man and a woman, so that they can live God's plan in fidelity to each other.
God's own communion of love with humanity is expressed in Revelation with the image of human marriage. Married love then, is an expression of the Covenant of love between God and His people. On the other hand, sin is an image of the infidelity of people to God. God however is always faithful - for this reason His love is the model for married love.