The Catholic Church teaches that human life and human sexuality are inseparable. Because Catholics believe God created human beings in his own image and likeness and that he found everything he created to be "very good," the Catholic Church teaches that human body and sex must likewise be good. The Church considers the expression of love between husband and wife to be an elevated form of human activity, joining husband and wife in complete, mutual self-giving, and opening their relationship to new life. As Pope Paul VI wrote in Humanae vitae, “The sexual activity, in which husband and wife are intimately and chastely united with one another, through which human life is transmitted, is, as the recent Council recalled, ‘noble and worthy.’” In cases in which sexual expression is sought outside sacramental marriage, or in which the procreative function of sexual expression within marriage is deliberately frustrated (e.g., the use of artificial contraception), the Catholic Church expresses grave moral concern.
The Church teaches that sexual intercourse has a purpose; and that outside marriage it is contrary to its purpose. According to the Catechism of the Catholic Church, "conjugal love ... aims at a deeply personal unity, a unity that, beyond union in one flesh, leads to forming one heart and soul", since the marriage bond is to be a sign of the love between God and humanity.
Among what are considered sins gravely contrary to chastity are masturbation, fornication, pornography, homosexual practices, and artificial contraception. Procurement of abortion, in addition to being considered grave matter, carries, under the conditions envisaged by canon law, the penalty of excommunication, "by the very commission of the offense".
Natural law (Latin: lex naturalis) is an ethical theory that posits the existence of a law whose content is set by nature and that therefore has validity everywhere. Despite pagan associations with natural law theory, a number (though not all) of the early Church Fathers sought to incorporate it into Christian theology.