“With the creation of man and woman in his own image and likeness, God crowns and brings to perfection the work of his hands: he calls them to a special sharing in his love and in his power as Creator and Father, through their free and responsible cooperation in transmitting the gift of human life…When, by means of recourse to periods (of fertility) and infertility, the couple respect the inseparable connection between the unitive and procreative meanings of human sexuality, they are acting as ministers of God’s plan and they benefit from their sexuality according to the original dynamism of total self-giving, without manipulation or alteration.” 
 (Pope John Paul II, On the Family, #28 & 32)