The Order of Procreation Before the Fall
St. Francis de Sales, Introduction to the Devout Life: Of the Sanctity of the Marriage Bed
Catholic Marrying Heretics: Forbidden and Condemned without a dispensation
The importance of Modesty (especially for women)
St. Francis de Sales, Introduction to the Devout Life: Decency in Attire
This article presents various teachings of the Church and Saints, in order to shed their light on a subject which, although it calls for much discretion, is nevertheless important to be understood rightly, from both a moral and from a doctrinal perspective.
St. Thomas Aquinas, Summa Theologica, Part I-II, Q. 34, Art. 1: "A sin, in human acts, is that which is against the order of reason. Now the order of reason consists in its ordering everything to its end in a fitting manner. Wherefore it is no sin if one, by the dictate of reason, makes use of certain things in a fitting manner and order for the end to which they are adapted, provided this end be something truly good. Now just as the preservation of the bodily nature of one individual is a true good, so, too, is the preservation of the nature of the human species a very great good. And just as the use of food is directed to the preservation of life in the individual, so is the use of venereal acts directed to the preservation of the whole human race. Hence Augustine says (De Bono Conjug. xvi): 'What food is to a man's well being, such is sexual intercourse to the welfare of the whole human race.' Wherefore just as the use of food can be without sin, if it be taken in due manner and order, as required for the welfare of the body, so also the use of venereal acts can be without sin, provided they be performed in due manner and order, in keeping with the end of human procreation."
Pope Eugene IV, Council of Florence, Session 11, ex cathedra: "[T]he one true God, Father, Son and holy Spirit, is the creator of all things that are, visible and invisible, who, when he willed it, made from his own goodness all creatures, both spiritual and corporeal, good indeed because they are made by the supreme good, but mutable because they are made from nothing, and it asserts that there is no nature of evil because every nature, in so far as it is a nature, is good."
That does not mean that all pleasure that one has in intercourse is good, however. Pleasure itself, being created by God is good, but remember that one can use created goods in a manner contrary to their natural end, contrary to reason, and that by doing so, one commits sin. This is a basic tenet of the Natural Law.
St. Augustine, Marriage and Concupiscence, 1:15:17: "I am supposing, then, although you are not lying [with your wife] for the sake of procreating offspring, you are not for the sake of lust obstructing their procreation by an evil prayer or an evil deed. Those who do this, although they are called husband and wife, are not; nor do they retain any reality of marriage, but with a respectable name cover a shame."
Once an intimate coupling has ended with a final deposit of seed, it is over. Anything done beyond this time would be for pleasure alone and not in line with the natural end of procreation. Any pleasure obtained in such a way would be unreasonable, disordered and evil pleasure.
Lactantius, Divine Institutes, 6:23:18: “The genital [‘generating’] part of the body, as the name itself teaches, has been received by us for no other purpose than the generation of offspring.”
St. Thomas Aquinas, Summa Theologica, Secunda Secundae Partis, Q. 154, A. 11: “(W)herever there occurs a special kind of deformity whereby the venereal act is rendered unbecoming, there is a determinate species of lust. This may occur in two ways: First, through being contrary to right reason, and this is common to all lustful vices; secondly, because, in addition, it is contrary to the natural order of the venereal act as becoming to the human race: and this is called "the unnatural vice." This may happen in several ways. First, by procuring pollution, without any copulation (i.e. self-stimulation, see next quotation from St. Thomas), for the sake of venereal pleasure: this pertains to the sin of "uncleanness" which some call "effeminacy." Secondly, by copulation with a thing of undue species, and this is called "bestiality." Thirdly, by copulation with an undue sex, male with male, or female with female, as the Apostle states (Romans 1:27): and this is called the "vice of sodomy." Fourthly, by not observing the natural manner of copulation (see the above quotes by St. Augustine and St. Barnabas), either as to undue means (i.e. procuring unnecessary and superfluous agents or devices), or as to other monstrous and bestial manners of copulation.”
St. Thomas Aquinas, Summa Theologica, Secunda Secundae Partis, Q. 148, Art. 6: "On the part of the body, mention is made of "uncleanness," which may refer either to the inordinate emission of any kind of superfluities, or especially to the emission of the semen. Hence a gloss on Ephesians 5:3, "But fornication and all uncleanness," says: "That is, any kind of incontinence that has reference to lust."
Do not forget that even willful venial sin is also odious and offensive to God, and if indulged persistently, can bring on spiritual blindness and lead to mortal sin.
Decree of the Holy Office under Pope Innocent XI, March 4, 1679: "The act of marriage exercised for pleasure only is entirely free of all fault and venial defect." - CONDEMNED (Denzinger 1159)
If a couple wishes to be perfect, they should pray to God to keep them from sinning during the act, and that they may beget holy children to love and serve God.
The Catechism of the Council of Trent, Sacrament of Matrimony
The Motives and Ends of Marriage: "We have now to explain why man and woman should be joined in marriage. First of all, nature itself by an instinct implanted in both sexes impels them to such companionship, and this is further encouraged by the hope of mutual assistance in bearing more easily the discomforts of life and the infirmities of old age.
"A second reason for marriage is the desire of family, not so much, however, with a view to leave after us heirs to inherit our property and fortune, as to bring up children in the true faith and in the service of God. That such was the principal object of the holy Patriarchs when they married is clear from Scripture. Hence the Angel, when informing Tobias of the means of repelling the violent assaults of the evil demon, says: I will show thee who they are over whom the devil can prevail; for they who in such manner receive matrimony as to shut out God from themselves and from their mind, and to give themselves to their lust, as the horse and mule which have not understanding, over them the devil hath power. He then adds: Thou shalt take the virgin with the fear of the Lord, moved rather for love of children than for lust, that in the seed of Abraham thou mayest obtain a blessing in children. It was also for this reason that God instituted marriage from the beginning; and therefore married persons who, to prevent conception or procure abortion, have recourse to medicine, are guilty of a most heinous crime nothing less than wicked conspiracy to commit murder.
"A third reason has been added, as a consequence of the fall of our first parents. On account of the loss of original innocence the passions began to rise in rebellion against right reason; and man, conscious of his own frailty and unwilling to fight the battles of the flesh, is supplied by marriage with an antidote by which to avoid sins of lust. For fear of fornication, says the Apostle, let every man have his own wife, and let every woman have her own husband; and a little after, having recommended to married persons a temporary abstinence from the marriage debt, to give themselves to prayer, he adds: Return together again, lest Satan tempt you for your incontinency.
"These are ends, some one of which, those who desire to contract marriage piously and religiously, as becomes the children of the Saints, should propose to themselves. If to these we add other causes which induce to contract marriage, and, in choosing a wife, to prefer one person to another, such as the desire of leaving an heir, wealth, beauty, illustrious descent, congeniality of disposition such motives, because not inconsistent with the holiness of marriage, are not to be condemned. We do not find that the Sacred Scriptures condemn the Patriarch Jacob for having chosen Rachel for her beauty, in preference to Lia."