The catechism (not suprisingly) gives an excellent summary of when a civil divorce is and is not morally permissible, and why. Those who don’t have the official catechism or compendium may be pleased to know that the Vatican has posted the text online.
A decree of nullity is a finding by a Church tribunal that a putative marriage was not validly ratified, consumated, or both. Marriages are all presumed to be valid until shown to be otherwise; tribunals do not investigate marriages still held to be binding by the state. There is also such a thing as a civil decree of nullity, but secular law differs on what constitutes a ratified and consumated marriage and the process and evidence required differs from that required for a civil divorce. Sometimes, a civil divorce accomplishes the same civil end in a more expedient manner. Therefore, the Church may rightly issue a decree of nullity when the state would not. The Church does not require couples to apply for civil annulments instead of a civil divorces, even where an annulment might have been possible.
The Church does not hold that civil divorce is always permissible, even in the absence of outside sexual relationships by the divorced spouses. Rather, marriage is a positive good to which the spouses must apply themselves to the best of their ability for their entire lives, where possible.
2382 The Lord Jesus insisted on the original intention of the Creator who willed that marriage be indissoluble.173 He abrogates the accommodations that had slipped into the old Law.174
Between the baptized, "a ratified and consummated marriage cannot be dissolved by any human power or for any reason other than death."175
2383 The separation of spouses while maintaining the marriage bond can be legitimate in certain cases provided for by canon law.176
If civil divorce remains the only possible way of ensuring certain legal rights, the care of the children, or the protection of inheritance, it can be tolerated and does not constitute a moral offense.
2384 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.177
2385 Divorce is immoral also because it introduces disorder into the family and into society. This disorder brings grave harm to the deserted spouse, to children traumatized by the separation of their parents and often torn between them, and because of its contagious effect which makes it truly a plague on society.
2386 *It can happen that one of the spouses is the innocent victim of a divorce decreed by civil law; this spouse therefore has not contravened the moral law. *There is a considerable difference between a spouse who has sincerely tried to be faithful to the sacrament of marriage and is unjustly abandoned, and one who through his own grave fault destroys a canonically valid marriage.178
2390 In a so-called free union, a man and a woman refuse to give juridical and public form to a liaison involving sexual intimacy.
The expression “free union” is fallacious: what can “union” mean when the partners make no commitment to one another, each exhibiting a lack of trust in the other, in himself, or in the future?
The expression covers a number of different situations: concubinage, rejection of marriage as such, or inability to make long-term commitments.182 All these situations offend against the dignity of marriage; they destroy the very idea of the family; they weaken the sense of fidelity. They are contrary to the moral law. *The sexual act must take place exclusively within marriage. Outside of marriage it always constitutes a grave sin and excludes one from sacramental communion. *
2391 Some today claim a “right to a trial marriage” where there is an intention of getting married later. However firm the purpose of those who engage in premature sexual relations may be, "the fact is that such liaisons can scarcely ensure mutual sincerity and fidelity in a relationship between a man and a woman, nor, especially, can they protect it from inconstancy of desires or whim."183 Carnal union is morally legitimate only when a definitive community of life between a man and woman has been established. Human love does not tolerate “trial marriages.” It demands a total and definitive gift of persons to one another.184 **