Things to be Aware of before Considering 'Divorce' as a result of Adultery

Adultery is the act of sexual intercourse with someone who is married to another. Adultery is defined as carnal connection between a married person and one unmarried, or between a married person and the spouse of another.
Divine Revelation
Exodus 20:14
You shall not commit adultery.
Deuteronomy 5:18 
You shall not commit adultery.
Leviticus 20:10
If a man commits adultery with his neighbor’s wife, both the adulterer and the
adulteress shall be put to death.
Proverbs 6:32
But he who commits adultery is a fool; he who would destroy himself does it.
Matthew 5:27
You have heard that it was said, ‘You shall not commit adultery.’  But I say to
you, everyone who looks at a woman with lust has already committed adultery
with her in his heart.
Matthew 5:32
But I say to you, whoever divorces his wife (unless the marriage is unlawful)
causes her to commit adultery, and whoever marries a divorced woman commits
adultery.
Matthew 15:19
For from the heart come evil thoughts, murder, adultery, unchastity, theft, false
witness, blasphemy.
Matthew 19:9
I say to you, whoever divorces his wife (unless the marriage is unlawful) and
marries another commits adultery.
Matthew 19:18
He asked him, “Which ones?” And Jesus replied, ” ‘You shall not kill; you shall
not commit adultery; you shall not steal; you shall not bear false witness;
Mark 10:19
You know the commandments: ‘You shall not kill; you shall not commit adultery;
you shall not steal; you shall not bear false witness; you shall not defraud; honor
your father and your mother.'”
Romans 13:9
The commandments, “You shall not commit adultery; you shall not kill; you shall
not steal; you shall not covet,” and whatever other commandment there may be,
are summed up in this saying, (namely) “You shall love your neighbor as
yourself.”
Catechism of the Catholic Church
Adultery refers to marital infidelity. When two partners, of whom at least one is married to another party, have sexual relations – even transient ones – they commit adultery. Christ condemns even adultery of mere desire.[Cf. Mt 5:27-28 .] The sixth commandment and the New Testament forbid adultery absolutely.[Cf. Mt 5:32 ; Mt 19:6 ; Mk 10:11 ; 1 Cor 6:9-10 .] The prophets denounce the gravity of adultery; they see it as an image of the sin of idolatry [Cf. Hos 2:7 ; Jer 5:7 ; Jer 13:27 ] . (2380)
Adultery is an injustice. He who commits adultery fails in his commitment. He does injury to the sign of the covenant which the marriage bond is, transgresses the rights of the other spouse, and undermines the institution of marriage by breaking the contract on which it is based. He compromises the good of human generation and the welfare of children who need their parents’ stable union. (2381)
Divorce is a grave offense against the natural law. It claims to break the contract, to which the spouses freely consented, to live with each other till death. Divorce does injury to the covenant of salvation, of which sacramental marriage is the sign. Contracting a new union, even if it is recognized by civil law, adds to the gravity of the rupture: the remarried spouse is then in a situation of public and permanent adultery: If a husband, separated from his wife, approaches another woman, he is an adulterer because he makes that woman commit adultery, and the woman who lives with him is an adulteress, because she has drawn another’s husband to herself.[St. Basil, Moralia 73, 1: PG 31, 849-852.] (2384)
Adultery, divorce, polygamy, and free union are grave offenses against the dignity of marriage. (2400)
The Magisterium of the Church
It follows therefore that they are destroying mutual fidelity, who think that the ideas and morality of our present time concerning a certain harmful and false friendship with a third party can be countenanced, and who teach that a greater freedom of feeling and action in such external relations should be allowed to man and wife, particularly as many (so they consider) are possessed of an inborn sexual tendency which cannot be satisfied within the narrow limits of monogamous marriage. That rigid attitude which condemns all sensual affections and actions with a third party they imagine to be a narrowing of mind and heart, something obsolete, or an abject form of jealousy, and as a result they look upon whatever penal laws are passed by the State for the preserving of conjugal faith as void or to be abolished. Such unworthy and idle opinions are condemned by that noble instinct which is found in every chaste husband and wife, and even by the light of the testimony of nature alone, – a testimony that is sanctioned and confirmed by the command of God: “Thou shalt not commit adultery,”[55] and the words of Christ: “Whosoever shall look on a woman to lust after her hath already committed adultery with her in his heart.”[56] The force of this divine precept can never be weakened by any merely human custom, bad example or pretext of human progress, for just as it is the one and the same “Jesus Christ, yesterday and today and the same for ever,”[57] so it is the one and the same doctrine of Christ that abides and of which no one jot or tittle shall pass away till all is fulfilled.[58]. Pope Pius XI, Casti Connubii, n. 73
The biblical Word of God several times urges the betrothed and the married to nourish and develop their wedlock by pure conjugal love and undivided affection.[10] Many men of our own age also highly regard true love between husband and wife as it manifests itself in a variety of ways depending on the worthy customs of various peoples and times. This love is an eminently human one since it is directed from one person to another through an affection of the will; it involves the good of the whole person, and therefore can enrich the expressions of body and mind with a unique dignity, ennobling these expressions as special ingredients and signs of the friendship distinctive of marriage. This love God has judged worthy of special gifts, healing, perfecting and exalting gifts of grace and of charity. Such love, merging the human with the divine, leads the spouses to a free and mutual gift of themselves, a gift providing itself by gentle affection and by deed; such love pervades the whole of their lives:[11] indeed by its busy generosity it grows better and grows greater. Therefore it far excels mere erotic inclination, which, selfishly pursued, soon enough fades wretchedly away. This love is uniquely expressed and perfected through the appropriate enterprise of matrimony. The actions within marriage by which the couple are united intimately and chastely are noble and worthy ones. Expressed in a manner which is truly human, these actions promote that mutual self-giving by which spouses enrich each other with a joyful and a ready will. Sealed by mutual faithfulness and hallowed above all by Christ’s sacrament, this love remains steadfastly true in body and in mind, in bright days or dark. It will never be profaned by adultery or divorce. Firmly established by the Lord, the unity of marriage will radiate from the equal personal dignity of wife and husband, a dignity acknowledged by mutual and total love. The constant fulfillment of the duties of this Christian vocation demands notable virtue. For this reason, strengthened by grace for holiness of life, the couple will painstakingly cultivate and pray for steadiness of love, large heartedness and the spirit of sacrifice. Authentic conjugal love will be more highly prized, and wholesome public opinion created about it if Christian couples give outstanding witness to faithfulness and harmony in their love, and to their concern for educating their children; also, if they do their part in bringing about the needed cultural, psychological and social renewal on behalf of marriage and the family. Especially in the heart of their own families, young people should be aptly and seasonably instructed in the dignity, duty and work of married love. Trained thus in the cultivation of chastity, they will be able at a suitable age to enter a marriage of their own after an honorable courtship. Vatican II, Gaudium et Spes, n. 49
Ecclesiastical Tradition
Hermas
“What then shall the husband do, if the wife continue in this disposition [adultery]? Let him divorce her, and let the husband remain single. But if he divorce his wife and marry another, he too commits adultery” (The Shepherd 4:1:6 [A.D. 80]).
Justin Martyr
“In regard to chastity, [Jesus] has this to say: ‘If anyone look with lust at a woman, he has already before God committed adultery in his heart.’ And, ‘Whoever marries a woman who has been divorced from another husband, commits adultery.’ According to our Teacher, just as they are sinners who contract a second marriage, even though it be in accord with human law, so also are they sinners who look with lustful desire at a woman. He repudiates not only one who actually commits adultery, but even one who wishes to do so; for not only our actions are manifest to God, but even our thoughts” (First Apology 15 [A.D. 151]).
Clement of Alexandria
“That Scripture counsels marriage, however, and never allows any release from the union, is expressly contained in the law: ‘You shall not divorce a wife, except for reason of immorality.’ And it regards as adultery the marriage of a spouse, while the one from whom a separation was made is still alive. ‘Whoever takes a divorced woman as wife commits adultery,’ it says; for ‘if anyone divorce his wife, he debauches her’; that is, he compels her to commit adultery. And not only does he that divorces her become the cause of this, but also he that takes the woman and gives her the opportunity of sinning; for if he did not take her, she would return to her husband” (Miscellanies 2:23:145:3 [A.D. 208]).
Origen
“Just as a woman is an adulteress, even though she seem to be married to a man, while a former husband yet lives, so also the man who seems to marry her who has been divorced does not marry her, but, according to the declaration of our Savior, he commits adultery with her” (Commentaries on Matthew 14:24 [A.D. 248]).
Council of Elvira
“Likewise, women who have left their husbands for no prior cause and have joined themselves with others, may not even at death receive Communion” (Canon 8 [A.D. 300]).
“Likewise, a woman of the faith [i.e., a baptized person] who has left an adulterous husband of the faith and marries another, her marrying in this manner is prohibited. If she has so married, she may not receive Communion—unless he that she has left has since departed from this world” (Canon 9).
“If she whom a catechumen [an unbaptized person studying the faith] has left shall have married a husband, she is able to be admitted to the fountain of baptism. This shall also be observed in the instance where it is the woman who is the catechumen. But if a woman of the faithful is taken in marriage by a man who left an innocent wife, and if she knew that he had a wife whom he had left without cause, it is determined that Communion is not to be given to her even at death” (Canon 10).
Basil the Great
“A man who marries after another man’s wife has been taken away from him will be charged with adultery in the case of the first woman; but in the case of the second he will be guiltless” (Second Canonical Letter to Amphilochius 199:37 [A.D. 375]).
Ambrose of Milan
“No one is permitted to know a woman other than his wife. The marital right is given you for this reason: lest you fall into the snare and sin with a strange woman. ‘If you are bound to a wife do not seek a divorce’; for you are not permitted, while your wife lives, to marry another” (Abraham 1:7:59 [A.D. 387]).
“You dismiss your wife, therefore, as if by right and without being charged with wrongdoing; and you suppose it is proper for you to do so because no human law forbids it; but divine law forbids it. Anyone who obeys men ought to stand in awe of God. Hear the law of the Lord, which even they who propose our laws must obey: ‘What God has joined together let no man put asunder’” (Commentary on Luke 8:5 [A.D. 389]).
Jerome
“Do not tell me about the violence of the ravisher, about the persuasiveness of a mother, about the authority of a father, about the influence of relatives, about the intrigues and insolence of servants, or about household [financial] losses. So long as a husband lives, be he adulterer, be he sodomite, be he addicted to every kind of vice, if she left him on account of his crimes, he is her husband still and she may not take another” (Letters 55:3 [A.D. 396]).
“Wherever there is fornication and a suspicion of fornication, a wife is freely dismissed. Because it is always possible that someone may calumniate the innocent and, for the sake of a second joining in marriage, act in criminal fashion against the first, it is commanded that when the first wife is dismissed, a second may not be taken while the first lives” (Commentaries on Matthew 3:19:9 [A.D. 398]).
Pope Innocent I
“[T]he practice is observed by all of regarding as an adulteress a woman who marries a second time while her husband yet lives, and permission to do penance is not granted her until one of them is dead” (Letters 2:13:15 [A.D. 408]).
Augustine
“Neither can it rightly be held that a husband who dismisses his wife because of fornication and marries another does not commit adultery. For there is also adultery on the part of those who, after the repudiation of their former wives because of fornication, marry others. This adultery, nevertheless, is certainly less serious than that of men who dismiss their wives for reasons other than fornication and take other wives. Therefore, when we say: ‘Whoever marries a woman dismissed by her husband for reason other than fornication commits adultery,’ undoubtedly we speak the truth. But we do not thereby acquit of this crime the man who marries a woman who was dismissed because of fornication. We do not doubt in the least that both are adulterers. We do indeed pronounce him an adulterer who dismissed his wife for cause other than fornication and marries another, nor do we thereby defend from the taint of this sin the man who dismissed his wife because of fornication and marries another. We recognize that both are adulterers, though the sin of one is more grave than that of the other. No one is so unreasonable to say that a man who marries a woman whose husband has dismissed her because of fornication is not an adulterer, while maintaining that a man who marries a woman dismissed without the ground of fornication is an adulterer. Both of these men are guilty of adultery” (Adulterous Marriages 1:9:9 [A.D. 419]).
“A woman begins to be the wife of no later husband unless she has ceased to be the wife of a former one. She will cease to be the wife of a former one, however, if that husband should die, not if he commit fornication. A spouse, therefore, is lawfully dismissed for cause of fornication; but the bond of chastity remains. That is why a man is guilty of adultery if he marries a woman who has been dismissed even for this very reason of fornication” (ibid., 2:4:4).
“Undoubtedly the substance of the sacrament is of this bond, so that when man and woman have been joined in marriage they must continue inseparably as long as they live, nor is it allowed for one spouse to be separated from the other except for cause of fornication. For this is preserved in the case of Christ and the Church, so that, as a living one with a living one, there is no divorce, no separation forever” (Marriage and Concupiscence 1:10:11 [A.D. 419]).
“In marriage, however, let the blessings of marriage be loved: offspring, fidelity, and the sacramental bond. Offspring, not so much because it may be born, but because it can be reborn; for it is born to punishment unless it be reborn to life. Fidelity, but not such as even the unbelievers have among themselves, ardent as they are for the flesh. . . . The sacramental bond, which they lose neither through separation nor through adultery, this the spouses should guard chastely and harmoniously” (ibid., 1:17:19).
The Teaching of the Fathers and Doctors of the Church\
St. Thomas, Summa II-II, 154, 9
I answer that, Adultery, as its name implies, “is access to another’s marriage-bed [ad alienum torum]” [Cf. Append. Gratian, ad can. Ille autem. xxxii, qu. 1. By so doing a man is guilty of a twofold offense against chastity and the good of human procreation. First, by accession to a woman who is not joined to him in marriage, which is contrary to the good of the upbringing of his own children. Secondly, by accession to a woman who is united to another in marriage, and thus he hinders the good of another’s children. The same applies to the married woman who is corrupted by adultery. Wherefore it is written (Sirach 23:32,33): “Every woman . . . that leaveth her husband . . . shall be guilty of sin. For first she hath been unfaithful to the law of the Most High” (since there it is commanded: “Thou shalt not commit adultery”); “and secondly, she hath offended against her husband,” by making it uncertain that the children are his: “thirdly, she hath fornicated in adultery, and hath gotten children of another man,” which is contrary to the good of her offspring. The first of these, however, is common to all mortal sins, while the two others belong especially to the deformity of adultery. Hence it is manifest that adultery is a determinate species of lust, through having a special deformity in venereal acts.
Human Reasoning
This is not a commandment of men, but one that comes directly from Jesus Christ. As Paul said, “To the married I give charge, not I but the Lord, that the wife should not separate from her husband (but if she does, let her remain single or else be reconciled to her husband)—and that the husband should not divorce his wife” (1 Cor. 7:10-11).
It only stands to reason that those engaging in the conjugal act must provide a context within which the potential child can be nurtured, and this context above all else entails a permanent commitment between those using their generative faculties in the conjugal act. Without this permanence, the dignity of the child is violated.
The child, first of all, is given a tremendous security by the permanent commitment of his parents. He knows that they will always be there, giving unconditional love, unless death causes the absence of one or both parents. Second, he learns a great deal from seeing this permanent commitment at work day in and day out. He learns the value of commitment.
One of the reasons so very many homosexual relationships do not have the character of permanence is because this particular reason or end for permanence is missing. True enough, permanence is a value in and of itself, irrespective of whether a child is present or not. But such inherent value of permanence is infused with deeper meaning when the child is present or potentially present: “Part of why we as a couple have a permanent commitment is precisely so that we can provide the best context for the nurturing of a new life.” (Envoy Magazine)
Adultery is the normal, but unlawful, use of sex by a married and a single person, or by two married persons, who, however, are not married to each other. This grievous sin is far worse than fornication, for it violates not only chastity, but it is a gross violation of justice (committed against the true spouse of the married party, or against both spouses of the married parties). Besides, it is a more damaging offense against the common good than fornication is. Summa Theologia-[IIa IIae], Questions 153 & 154: