This encyclical published by John Paul II in 1995 argues vigorously against situation ethics and proportionalism.  Instead, it is a defence of Aquinas’ view that some actions are intrinisically good or evil by virtue of their object (their aim or purpose eg sex has as its divine purpose reproduction so use of contracpetion is intrinisally evil). It’s worth trying to read the whole document and to make sense of the argument, click here for full text.  Notice that there is a teleological element to thsi argument (see “ordered to its final end” below) but the teleology is very different to situation ethics and utilitarian teleology.  See my handout on natural alw if you want to understand fully this important distinction and how teleology may be used in different ways.

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.
82. Furthermore, an intention is good when it has as its aim the true good of the person in view of his ultimate end. But acts whose object is “not capable of being ordered” to God and “unworthy of the human person” are always and in every case in conflict with that good. Consequently, respect for norms which prohibit such acts and oblige semper et pro semper, that is, without any exception, not only does not inhibit a good intention, but actually represents its basic expression.

The doctrine of the object as a source of morality represents an authentic explicitation of the Biblical morality of the Covenant and of the commandments, of charity and of the virtues. The moral quality of human acting is dependent on this fidelity to the commandments, as an expression of obedience and of love. For this reason – we repeat – the opinion must be rejected as erroneous which maintains that it is impossible to qualify as morally evil according to its species the deliberate choice of certain kinds of behaviour or specific acts, without taking into account the intention for which the choice was made or the totality of the foreseeable consequences of that act for all persons concerned. Without the rational determination of the morality of human acting as stated above, it would be impossible to affirm the existence of an “objective moral order” and to establish any particular norm the content of which would be binding without exception. This would be to the detriment of human fraternity and the truth about the good, and would be injurious to ecclesial communion as well.

83. As is evident, in the question of the morality of human acts, and in particular the question of whether there exist intrinsically evil acts, we find ourselves faced with the question of man himself, of his truth and of the moral consequences flowing from that truth. By acknowledging and teaching the existence of intrinsic evil in given human acts, the Church remains faithful to the integral truth about man; she thus respects and promotes man in his dignity and vocation. Consequently, she must reject the theories set forth above, which contradict this truth.