Encyclical Letter of Pope Paul VI:
The transmission of human life is a most serious role in which married people collaborate freely and responsibly with God the Creator. It has always been a source of great joy to them, even though it sometimes entails many difficulties and hardships.
The fulfillment of this duty has always posed problems to the conscience of married people, but the recent course of human society and the concomitant changes have provoked new questions. The Church cannot ignore these questions, for they concern matters intimately connected with the life and happiness of human beings… READ FULL TEXT