Re: Why must sex be both unitive and procreative?
In an age in which sex is considered to be one way among others to pass the time, with or without a partner, it indeed might be difficult for some to understand the Church's position because, as Catholic apologist Frank Sheed once said, "Modern man practically never thinks about sex" (emphasis added). Modern man may do a lot of things with sex, but thinking about it on a serious intellectual level usually is not one of them. But the Church looks at the purposes for which the act was intended, which are to bind together husband and wife and to procreate children, and says that any action that negates or destroys one or both of those two purposes of sex (which the Church prefers to call marital relations) is a sinful act. To take your example, contraception not only blocks conception, it also blocks husband and wife from completely bonding -- something that is impossible when one or both partners has blocked out the other or mutilated his or her reproductive capabilities.