From: Matthew 5:27-32
Jesus and His Teaching, the Fulfillment of the Law (Continuation)
[Jesus said to His disciples:)  "You have heard that it was said, `You shall not commit adultery.'  But I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart.  If your right eye causes you to sin, pluck it out and throw it away; it is better that you lose one of your members than that your whole body be thrown into hell.  And if your right hand causes you to sin, cut it off and throw it away; it is better that you lose one of your members than that your whole body go into hell.
 "It was also said, `Whoever divorces his wife, let him give her a certificate of divorce.'  But I say to you that every one who divorces his wife, except on the ground of unchastity, makes her an adulteress; and whoever marries a divorced woman commits adultery."
27-30. This refers to a sinful glance at any woman, be she married or not. Our Lord fills out the precepts of the Old Law, where only adultery and the coveting of one's neighbor's wife were considered sinful.
"Lustfully": feeling is one thing, consenting another. Consent presupposes that one realizes the evil of these actions (looking, imagining, having impure thoughts) and freely engages in them.
Prohibition of vices always implies a positive aspect--the contrary virtue. Holy purity, like every other virtue, is something eminently positive; it derives from the First Commandment and is also directed to it: "You shall love the Lord your God with all your heart, with all your soul, and with all your mind" (Matthew 22:37). "Purity is a consequence of the love that prompts us to commit to Christ our soul and body, our faculties and senses. It is not something negative; it is a joyful affirmation" ([St] J. Escriva, "Christ Is Passing By", 5). This virtue demands that we use all the resources available to us, to the point of heroism if necessary.
"Right eye", "right hand", refers to whatever we value most. Our Lord lays it on the line and it not exaggerating. He obviously does not mean that we should physically mutilate ourselves, but that we should fight hard without making any concessions, being ready to sacrifice anything which clearly could put us in the way of offending God. Jesus' graphic words particularly warn us about one of the most common occasions of sin, reminding us of how careful we need to be guarding our sight. King David, by indulging his curiosity, went on to commit adultery and crime. He later wept over his sins and led a holy life in the presence of God (cf. 2 Samuel 11 and 12).
"The eyes! Through them many iniquities enter the soul. So many experiences like David's!--If you guard your sight you will have assured the guard of your heart: ([St] J. Escriva, "The Way", 183).
Among the ascetical methods of protecting the virtue of holy purity are: frequent Confession and Communion; devotion to our Lady; a spirit of prayer and mortification; guarding of the senses; flight from occasions of sin; and striving to avoid idleness by always being engaged in doing useful things. There are two further means which are particularly relevant today: "Decorum and modesty are younger brothers of purity" ([St] J. Escriva, "The Way", 128). Decorum and modesty are a sign of good taste, of respect for others and of human and Christian dignity. To act in accord with this teaching of our Lord, the Christian has to row against the current in a paganized environment and bring his influence for good to bear on it.
"There is need for a crusade of manliness and purity to counteract and undo the savage work of those who think that man is a beast. And that crusade is a matter for you" ([St] J. Escriva, "The Way", 121).
31-32. The Law of Moses (Deuteronomy 24:1), which was laid down in ancient times, had tolerated divorce due to the hardness of heart of the early Hebrews. But it had not specified clearly the grounds on which divorce might be obtained. The rabbis worked out different sorts of interpretations, depending on which school they belonged to--solutions ranging from very lax to quite rigid. In all cases, only husband could repudiate wife, not vice-versa. A woman's inferior position was eased somewhat by the device of a written document whereby the husband freed the repudiated woman to marry again if she wished. Against these rabbinical interpretations, Jesus re-establishes the original indissolubility of marriage as God instituted it (Genesis 1:27; 2:24; cf. Matthew 19:4-6; Ephesians 1:31; 1 Corinthians 7:10).
[The RSVCE carries a note which reads: "unchastity": The Greek word used here appears to refer to marriages which were not legally marriages, because they were within the forbidden degrees of consanguinity (Leviticus 18:6-16) or contracted with a Gentile. The phrase "except on the ground of unchastity" does not occur in the parallel passage in Luke 16:18. See also Matthew 19:9 (Mark 10:11-12), and especially 1 Corinthians 7:10-11, which shows that the prohibition is unconditional.] The phrase, "except on the ground of unchastity", should not be taken as indicating an exception to the principle of absolute indissolubility of marriage which Jesus has just re-established. It is almost certain that the phrase refers to unions accepted as marriage among some pagan people, but prohibited as incestuous in the Mosaic Law (cf. Leviticus 18) and in rabbinical tradition. The reference, then, is to unions radically invalid because of some impediment. When persons in this position were converted to the True Faith, it was not that their union could be dissolved; it was declared that they had never in fact been joined in true marriage. Therefore, this phrase does not do against the indissolubility of marriage, but rather reaffirms it.
On the basis of Jesus' teaching and guided by the Holy Spirit, the Church has ruled that in the specially grave case of adultery it is permissible for a married couple to separate, but without the marriage bond being dissolved; therefore, neither party may contract a new marriage.
The indissolubility of marriage was unhesitatingly taught by the Church from the very beginning; she demanded practical and legal recognition of this doctrine, expounded with full authority by Jesus (Matthew 19:3-9; Mark 10:1-12; Luke 16:18) and by the Apostles (1 Corinthians 6:16; 7:10-11; 39; Romans 7:2-3; Ephesians 5:31f). Here, for example, are just a few texts from the Magisterium on this subject:
"Three blessings are ascribed to matrimony [...]. The third is the indissolubility of matrimony--indissoluble because it signifies the indivisible union of Christ with the Church. Although a separation from bed may be permitted by reason of marital infidelity, nevertheless it is not permitted to contract another matrimony since the bond of a marriage lawfully contracted is perpetual" (Council of Florence, "Pro Armeniis").
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.