St. Francis de Sales, in his book, "Introduction to the Devout Life", speaks of marital chastity.
St. Francis: "The marriage bed should be spotless (Heb. 13:4) as the Apostle says, that is, free from immodesty and other unholy defilements. Also, holy Marriage was first established in the earthly Paradise. There was no disorder of concupiscence, nor anything immodest till the fall." 
Incompatible with this spotless marriage bed, which is free from immodesty and unholy defilements, is any type of sex toy, or "marital aid" as it is euphemistically called. In addition, any sexual act that is not inherently unitive and procreative is also a defilement. For the purpose of sex is procreation.
St. Francis: "Eating is directed to the preservation of life. Eating, simply for the nourishment and preservation of life, is good, holy and recommended. So too acts required for the generation children and the multiplication of persons in marriage are good and very holy, since this is the principal purpose of marriage." 
The principle purpose of marriage is the generation and education of children; the principle purpose of marital relations is precisely this generation (i.e. procreation). Each and every sexual act in a marriage must be unitive and procreative; otherwise, the act is intrinsically evil and always gravely immoral. For sex is ordained by God for the purpose of procreation.
St. Francis: "Similarly the marriage debt should always be rendered faithfully, frankly and just as if it were with the hope of begetting children, even though sometimes there may be no such hope." 
What if the procreation of children, through no fault of the spouses, is not possible? Even then, the sexual acts must remain ordered toward the unitive and procreative ends. For, in the case of every intrinsically evil act, it is not the attainment of the evil object that makes the act intrinsically evil, but rather the deliberate knowing choice of an act ordered toward that object (that end). If a person chooses the act of murder, but the murder fails to result in the death of the innocent, the act was nevertheless intrinsically evil. For the person chose the disordered act. Similarly, an act of contracepted sex remains immoral, if the contraception fails and a child is conceived. And an act of marital relations open to life remains moral, if the act fails to attain the procreative end. As long as the marital sexual act is of the natural type, which has the proper ordering toward the unitive, and procreative ends, it is inherently moral. Then, too, every unnatural sexual act is inherently disordered, and so it is always wrong to knowingly choose, even in marriage.
St. Francis: "In fact, marital intercourse which is so holy, so just, so commendable and so useful to society is nevertheless dangerous on certain occasions to those who exercise it. Sometimes, it makes them die by mortal sin. This happens when the order established for the generation of children is violated and perverted. In such cases, in so far as one is more or less led astray from this order, the sins become more or less detestable but always mortal. Since the procreation of children is the primary and principal end of marriage, it is never lawful to turn away from the required order. Nevertheless, it is not possible to effect it in some particular circumstances as in the case of barrenness or pregnancy which prevents begetting and generation. In such case sexual intercourse does not cease to be just and holy provided the laws of generation are followed." 
The "order established for the generation of children" refers to the ordering of a sexual act toward the end of procreation. Natural sexual acts are open to life. Unnatural sexual acts are inherently non-unitive and non-procreative. When the married couple use an unnatural sexual act -- even as so-called foreplay before or after natural relations -- they violate and pervert the natural order established by God for marriage and marital relations. Such sins are "always mortal". This is true because "the procreation of children is the primary and principal end of marriage." Unnatural sexual acts represent a deliberate turning away from "the required order".
What happens in the case of "barrenness or pregnancy", when the procreative end cannot be attained? It is not the attainment of the moral object that makes an act inherently good or inherently evil, but rather the choice of the act ordered toward that good or evil end. So sexual intercourse in marriage remains "just and holy", but only if "the laws of generation are followed" -- meaning only if the sexual acts are each unitive and procreative. Unnatural sexual acts are never justified, neither as an isolated act, nor in combination with a moral act of natural relations. Intrinsically evil acts are not justified by intention, circumstances, or other acts.
St. Francis: "No casual circumstance can ever go against the law which governs the principal aim of marriage. Indeed, the shameful and detestable action which Onan committed in his marriage was abominable before God, so says the Sacred Scripture in the thirty-eighth chapter of Genesis. Some heretics of our age, a hundred times more blameworthy than the Cynics of whom St. Jerome speaks in his commentary on the Letter to the Ephesians, hold that it was the depraved intention of this wicked man that displeased God. Scripture, however, speaks quite otherwise, and declares specifically that the act itself which he committed was detestable and abominable in the sight of God." 
The sin of Onan was to choose a type of sexual relations, within marriage, which was deprived of its unitive and procreative ends. The sexual act of withdrawal begins as natural marital relations open to life -- but then the husband sins gravely by completing the act without union as one flesh with his wife, thereby thwarting the procreative end as well. The sin of Onan was, in one sense, a type of contraception. But in another sense, it was an unnatural sexual act. For sexual acts are unnatural when they are not ordered toward the unitive and procreative ends.
And it did not matter that Onan was validly married to his wife. The sin remains gravely immoral as all three ends are required in each and every sexual act: marital, unitive, and procreative.
The USCCB Catechism teaches on sex within marriage:
"Each and every sexual act in a marriage needs to be open to the possibility of conceiving a child." 
It is not sufficient for some acts within a set of sexual acts to be unitive and procreative. Each and every sexual act in a marriage must have both meanings, the unitive and the procreative. To be moral, a sexual act must be marital, unitive, and procreative.
A pastoral letter from the U.S. Bishops, Marriage: Love and Life in the Divine Plan, elaborates further:
"Sometimes one hears it said that as long as the marriage as a whole is open to children, each individual act of intercourse need not be. In fact, however, a marriage is only as open to procreation as each act of intercourse is, because the whole meaning of marriage is present and signified in each marital act. Each marital act signifies, embodies, and renews the original and enduring marital covenant between husband and wife. That is what makes intercourse exclusively a marital act." 
In other words, moral sexual intercourse only occurs in marriage, and each single marital act (sexual act in a marriage) must be unitive and procreative. Therefore, a sexual act is not justified, if it is inherently non-unitive or non-procreative or non-marital. All three of these moral objects must be present -- marital, unitive, and procreative -- for a sexual act to be moral.
St. Francis: "Chastity is the lily of virtues. It renders men almost equal to the Angels. Nothing is beautiful except by purity and the purity of men is chastity. We term chastity as honesty and the profession of it honour. It is called integrity and its opposite corruption. In short it has its own glory all apart to be the beautiful and fair virtue of the soul and body."
Sexual sins within marriage exist. Such sins are "against the laws of marriage", which includes the unitive and procreative ends of sexual relations. All non-unitive and non-procreative sexual acts are "forbidden and prohibited". For marriage is not a justification for grave sexual sins.
St. Francis: "But as regards those who are married, it is a fact that chastity is very necessary for them, even though common people may not think so. For the married, chastity does not consist in abstaining completely from carnal pleasures but observing moderation amidst such pleasures. Now this commandment, Be angry and do not sin (Ps. 4:4) is, in my opinion, more difficult than the other: do not be angry at all. Avoiding anger is easier than regulating it. Even so, it is easier to keep oneself away from carnal pleasures than to keep moderation in them. It is true that the holy liberty of marriage has a special power to extinguish the fire of concupiscence, but the weakness of those who enjoy them passes easily from permission to license and from use to abuse." 
Moderation within marital relations certainly includes abstaining from unnatural sexual acts, from the use of sex toys, from masturbation (of one's self or one's spouse), and from the unnatural sexual acts. Such sins are an abuse of holy matrimony. To claim that grave sexual sins are justified by the Sacrament of Marriage is a grave heresy.
St. Francis: "Just as we see many rich people steal, not due to necessity but avarice, so also we see many married people given up to intemperance and lewdness in spite of the lawful object at which they should and could stop. Their concupiscence is like a wild fire which burns here and there without limiting itself to any one place. It is always dangerous to take very strong medicines because if one takes more than is necessary, or if it is not well-prepared, much harm is done. Marriage was blessed and ordained partly as a remedy for concupiscence. Undoubtedly, it is a very good remedy but nonetheless very strong and, as a consequence, very dangerous if it is not reasonably used." 
The lawful object of marital relations is the marital, unitive, and procreative ends. When these ends are present in the moral object of the sexual act -- the natural marital act -- then the enjoyment of sexual pleasure is licit. But "many married people" today have given themselves up "to intemperance and lewdness". They seek mainly and only sexual pleasure. They obtain that pleasure by any means they wish, without regard for God and morality. They have fallen into dangerous sins.
St. Francis: "I add that a variety of human affairs, besides long sickness, often separate the husbands from their wives. Therefore the married need two kinds of chastity: one of absolute abstinence, when they are separated occasionally as I have just mentioned; the other of moderation when they are together in their day to day life. Indeed St. Catherine of Sienna saw among the damned many souls greatly tormented for violating the sanctity of marriage. What happened, she said, was not due to the mere seriousness of the sin, for murders and blasphemies are even more enormous, but in so far as those who commit it do not make it a matter of conscience and consequently continue in it for a long time." 
At Fatima, the Blessed Virgin Mary revealed that more souls go to Hell for sins of the flesh than for any other reason. St. Catherine of Siena adds that many of the damned are "greatly tormented" for sexual sins within marriage. And we see this very fault today among Catholics who publicly proclaim the grave error that unnatural sexual acts are somehow moral to use as foreplay or in conjunction with an act of natural marital relations. They continue in this type of sin many times for a long period, and so it will be the cause of the majority of their torments in Hell, if they do not repent.
More Reading: The Catholic Marriage Bed
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