From the Compendium of the Catechism of the Catholic Church:
THE SIXTH COMMANDMENT:
YOU SHALL NOT COMMIT ADULTERY
487. What responsibility do human persons have in regard to their own sexual identity?
God has created human beings as male and female, equal in personal dignity, and has called them to a vocation of love and of communion. Everyone should accept his or her identity as male or female, recognizing its importance for the whole of the person, its specificity and complementarity.
488. What is chastity?
Chastity means the positive integration of sexuality within the person. Sexuality becomes truly human when it is integrated in a correct way into the relationship of one person to another. Chastity is a moral virtue, a gift of God, a grace, and a fruit of the Holy Spirit.
489. What is involved in the virtue of chastity?
The virtue of chastity involves an apprenticeship in self-mastery as an expression of human freedom directed towards self-giving. An integral and continuing formation, which is brought about in stages, is necessary to achieve this goal.
490. What are the means that aid the living of chastity?
There are many means at one’s disposal: the grace of God, the help of the sacraments, prayer, self-knowledge, the practice of an asceticism adapted to various situations, the exercise of the moral virtues, especially the virtue of temperance which seeks to have the passions guided by reason.
491. In what way is everyone called to live chastity?
As followers of Christ, the model of all chastity, all the baptised are called to live chastely in keeping with their particular states of life. Some profess virginity or consecrated celibacy which enables them to give themselves to God alone with an undivided heart in a remarkable manner. Others, if they are married live in conjugal chastity, or if unmarried practise chastity in continence.
492. What are the principal sins against chastity?
Grave sins against chastity differ according to their object: adultery, masturbation, fornication, pornography, prostitution, rape, and homosexual acts. These sins are expressions of the vice of lust. These kinds of acts committed against the physical and moral integrity of minors become even more grave.
493. Although it says only “you shall not commit adultery” why does the sixth commandment forbid all sins against chastity?
Although the biblical text of the Decalogue reads “you shall not commit adultery” (Exodus 20:14), the Tradition of the Church comprehensively follows the moral teachings of the Old and New Testaments and considers the sixth commandment as encompassing all sins against chastity.
494. What is the responsibility of civil authority in regard to chastity?
Insofar as it is bound to promote respect for the dignity of the person, civil authority should seek to create an environment conducive to the practice of chastity. It should also enact suitable legislation to prevent the spread of the grave offenses against chastity mentioned above, especially in order to protect minors and those who are the weakest members of society.
495. What are the goods of conjugal love to which sexuality is ordered?
The goods of conjugal love, which for those who are baptized is sanctified by the sacrament of Matrimony, are unity, fidelity, indissolubility, and an openness to the procreation of life.
496. What is the meaning of the conjugal act?
The conjugal act has a twofold meaning: unitive (the mutual self-giving of the spouses) and procreative (an openness to the transmission of life). No one may break the inseparable connection which God has established between these two meanings of the conjugal act by excluding one or the other of them.
497. When is it moral to regulate births?
The regulation of births, which is an aspect of responsible fatherhood and motherhood, is objectively morally acceptable when it is pursued by the spouses without external pressure; when it is practiced not out of selfishness but for serious reasons; and with methods that conform to the objective criteria of morality, that is, periodic continence and use of the infertile periods.
498. What are immoral means of birth control?
Every action – for example, direct sterilization or contraception – is intrinsically immoral which (either in anticipation of the conjugal act, in its accomplishment or in the development of its natural consequences) proposes, as an end or as a means, to hinder procreation.
499. Why are artificial insemination and artificial fertilization immoral?
They are immoral because they dissociate procreation from the act with which the spouses give themselves to each other and so introduce the domination of technology over the origin and destiny of the human person. Furthermore, heterologous insemination and fertilization with the use of techniques that involve a person other than the married couple infringe upon the right of a child to be born of a father and mother known to him, bound to each other by marriage and having the exclusive right to become parents only through each another.
500. How should children be considered?
A child is a gift of God, the supreme gift of marriage. There is no such thing as a right to have children (e.g. “a child at any cost”). But a child does have the right to be the fruit of the conjugal act of its parents as well as the right to be respected as a person from the moment of conception.
501. What can spouses do when they do not have children?
Should the gift of a child not be given to them, after exhausting all legitimate medical options, spouses can show their generosity by way of foster care or adoption or by performing meaningful services for others. In this way they realize a precious spiritual fruitfulness.
502. What are the offenses against the dignity of marriage?
These are: adultery, divorce, polygamy, incest, free unions (cohabitation, concubinage), and sexual acts before or outside of marriage.