From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
In teaching the existence of intrinsically evil acts, the Church accepts the teaching of Sacred Scripture. The Apostle Paul emphatically states: “Do not be deceived: neither the immoral, nor idolaters, nor adulterers, nor sexual perverts, nor thieves, nor the greedy, nor drunkards, nor revilers, nor robbers will inherit the Kingdom of God” (1 Cor 6:9-10).
If acts are intrinsically evil, a good intention or particular circumstances can diminish their evil, but they cannot remove it. They remain “irremediably” evil acts; per se and in themselves they are not capable of being ordered to God and to the good of the person. “As for acts which are themselves sins (cum iam opera ipsa peccata sunt), Saint Augustine writes, like theft, fornication, blasphemy, who would dare affirm that, by doing them for good motives (causis bonis), they would no longer be sins, or, what is even more absurd, that they would be sins that are justified?”.134
Consequently, circumstances or intentions can never transform an act intrinsically evil by virtue of its object into an act “subjectively” good or defensible as a choice.
134 Contra Mendacium, VII, 18: Patrologiæ Cursus completus, Series Latina 40, 528; cf. Saint Thomas Aquinas, Quæstiones Quodlibetales, IX, q. 7, a. 2; Catechism of the Catholic Church, Nos. 1753-1755.
— Veritatis Splendor 81