The MOST Theological Collection: A Basic Catholic Catechism
"Part VIII: Commandments VI and IX"
Sixth and Ninth Commandments: "You shall not commit adultery. You shall not covet your neighbor's wife."
1. Sins against Chastity
The sixth and ninth commandments forbid us to try to seek out sexual pleasure or to accept it when temptation offers it, outside of marriage. St. Paul told the licentious Corinthians (1 Cor 6. 19-20: "Do you not know that your body is the temple of the Holy Spirit, whom you have from God, and you are not your own. For you have been bought at a price [the price of redemption]."
Not only external acts which by nature are apt to arouse sexual pleasure themselves are forbidden outside of marriage; also thoughts and desires deliberately aroused, or accepted when they come unbidden are sinful. But a good person may have a long siege of such thoughts: as long as he/she tries to get rid of them each time he/she notices them (there can be distractions), there will at least be no mortal sin. Sometimes when one is occupied with something else, such a thought may slip into the brain and unroll itself like a movie. It may run some time until a sort of wake-up point where the person says to self: I must not have this" and then gets busy against it. Up to that point there is never mortal sin.
Masturbation turns one back into the shell of self in which one started life, and so makes real love difficult, gives a poor forecast for success in marriage.
Homosexual acts are most gravely wrong, and the more so if the sinner asserts they are good. St. Paul painted a sad picture of the vices of the gentiles in Romans chapter 1, and made homosexuality the centerpiece. He added (1:32) that the lowest depth is to not only sin, but to call sin good.
It is not a sin to have homosexual temptations, provided one does not give into them.
Contraception is really only mutual masturbation. For the use of sex is divinely ordained first of all to propagate the race. To deliberately rule that out is to fly directly into the face of God's plan. Experience shows that where contraception is common, abortion tends to follow as a sort of backup.
The same is not to be said of Natural Family Planning: it makes legitimate use of the characteristics God Himself has built into our nature. Its reliability is over 99%, as high as that of artificial methods. And there are no bad side effects.
Experience shows that to use it strengthens marriage. The reason is this: any pleasure, even sex, can grow dull if we take it very constantly. Some small abstention, needed for NFP, helps to revive the pleasure of lawful sex.
Fornication is having sex outside marriage when both parties are unmarried. Adultery is the same except that one or both are married to someone else. It is a violation not only of chastity but of justice towards the spouse.
2. True Conjugal Love
Vatican II has taught (Constitution on the Church in the Modern World §49): "The actions by which the spouses are intimately and chastely united are honorable and worthy, and if done in true human fashion, signify and promote the self-giving by which the couple gladly and gratefully enrich each other." They can even be meritorious if done in accordance with God's plan. The same document added (§50): "Marriage and conjugal love are by their nature ordained to the procreation and education of offspring." We notice there are two purposes, procreation, and mutual love. Both are intended by God, yet in such a way that the procreation is primary, since the promotion of love "is by nature ordained to" procreation, i.e., is secondary to it.
It is only in marriage that children can receive the formation, love and care that they need. This is why sex outside of marriage is so wrong.
To separate the two functions, procreative and unitive is wrong, e.g., in test-tube babies.
Carried out according to Our Father's plan, marriage can lead to real growth in holiness, as we shall explain later on.
3. The Means to Chastity
Much prayer, especially to the Blessed Mother (particularly her Rosary) is needed along with keeping watch over what one sees, reads, dreams about. The thought of death and judgment helps greatly, and frequent use of the Sacrament of Penance and the Holy Eucharist are important.
Mortification of the body, by giving up legitimate things is practically indispensable as a help. At a time when temptation is strong, it helps to get into the company of others (unless it be the person of the opposite sex who is the cause of temptation). If that is not possible, getting occupied with something that readily holds one's interest, such as absorbing reading is important. The more we get our attention onto anything else, the less power the temptation has. Many find it helpful to say to themselves: "I will keep pure just this one time." They do not mean to fall the next time, but this is a way of lightening the psychological difficulty.
It is important to realize and to talk it through with a prospective mate, that since love is the desire for the well-being and happiness of another for the other's sake, to use the other for sensory gratification is not love. It is closer to the opposite, for it puts both into such a state that if death should come, the person would never be happy again, would be eternally wretched. That is the opposite of willing well-being to another. And real love is rather unlikely to develop when two indulge in much premarital sex. It will feel like love, but will only be chemistry.