The Catechism of the Catholic Church:

2380 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. The sixth commandment and the New Testament forbid adultery absolutely. The prophets denounce the gravity of adultery; they see it as an image of the sin of idolatry.

2381 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.

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2382 . . . . Between the baptized, “a ratified and consummated marriage cannot be dissolved by any human power or for any reason other than death.”

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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.

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.  

Rudy Giuliani’s public behavior and flagrant disregard of the Church’s teaching on adultery and divorce:

Giuliani informed his second wife, Donna Hanover, of his intention to seek a separation in a 2000 press conference. The announcement was precipitated by a tabloid frenzy after Giuliani marched with his then-mistress, Judith Nathan, in New York’s St. Patrick’s Day parade, an acknowledgement of infidelity so audacious that Daily News columnist Jim Dwyer compared it with “groping in the window at Macy’s.” In the acrid divorce proceedings that followed, Hanover accused Giuliani of serial adultery, alleging that Nathan was just the latest in a string of mistresses, following an affair the mayor had had with his former communications director [Washington Monthly].

“If there’s going to be a divorce, let’s have the truth about why — Rudy’s open and notorious adultery,” [Donna Hanover, Giuliani's second wife]

*Administrative note: We have not yet decided whether to keep this as an issue page or not. Initially, we decided that CAR would only discuss Mayor Giuliani’s public-policy positions; but the public disregard (indeed, complete contempt) he showed for his second marriage, and in turn the Church’s teachings on adultery and divorce, have caused us to seriously rethink whether the failure to include these issues is entirely appropriate. It’s a difficult call. And as such,we would greatly appreciate any feedback our supporters might wish to provide on this matter.