There are three fonts of morality: (1) intention, (2) moral object, (3) circumstances. The Magisterium definitively teaches (Veritatis Splendor, Catechism of the Catholic Church, etc.) that to be moral an act must have three good fonts. Any one bad font makes the act immoral and therefore sinful.
“The principle of double effect is the natural result of a proper understanding of the three fonts of morality; it is not additional to the three fonts, but rather is found within these fonts. Under the principle of double effect, an act is moral if the first two fonts are good and if, in the third font, the good outweighs the bad. When the intention, including the intended means and the intended end, is good, and when the act itself is good (due to a good moral object), then the morality of the overall act depends on the circumstances. But if the moral weight of the circumstances depends on two effects (i.e. consequences) of the act, one good and the other bad, the act is moral if the good outweighs the bad, and immoral if the bad outweighs the good.”
“So the principle of double effect is nothing other than the proper application of the three fonts of morality to a particular class of acts, those in which the first two fonts are good, and in which the third font has both good and bad consequences (i.e. effects). This principle also applies if there are multiple good and multiple bad consequences. In any case, as long as the first two fonts are good and, in the third font, the totality of the good consequences outweighs the totality of the bad consequences, the act is moral. As long as the third font has good and bad circumstances, and the good outweighs the bad, and the other two fonts are good, then the act is justified under the principle of double effect. If all three fonts are good, and there is nothing bad in the circumstances, then the act would be moral; but this would not be classified under the principle of double effect, if there are only good effects.”
Quoted from my book, The Catechism of Catholic Ethics, n. 061.
Whenever the moral object of an act is evil, that act is intrinsically evil and always immoral, regardless of the other two fonts (intention, circumstances).
“But no reason, however grave, may be put forward by which anything intrinsically against nature may become conformable to nature and morally good.” (Pope Pius XI, Casti Connubii, n. 54)
Pope John Paul II: “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…. 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.” (Veritatis Splendor, n. 81).
The prinicple of double effect NEVER justifies an intrinsically evil act.