An excellent exposition of these principles is found in Norman L. Geisler’s Christian Ethics: Options and Issues, (Grand Rapids: Baker Books, 1989), pages 97-100:
What about moral conflicts? What should one do when two or more of his absolute obligations come into unavoidable conflict?
Basically, there are three answers to this question. First, unqualified absolutism [Roman Catholicism favors this one] affirms that all such conflicts are only apparent; they not real. In short, no two absolute obligations ever come into unavoidable conflict. Second, conflicting absolutism [the more Lutheran position in my opinion] admits to real moral conflicts but claims that one is guilty no matter which way he goes. Third, graded absolutism (or the greater-good position) [favored by Calvinists] agrees with the view that real moral conflicts do sometimes occur, but maintains that one is personally guiltless if he does the greatest good...
The central assumption of the ethical position of conflicting absolutism is that we live in a fallen world, and in such a world real moral conflicts do occur. The accompanying premise, however, is that when two duties conflict, man is morally responsible to both duties. God’s law can never be broken without guilt. In such cases, therefore, one must simply do the lesser evil, confess his sin, and ask for God's forgiveness.
Conflicting absolutism has roots in the Greek world, was incorporated into Reformation thinking, and finds expression in both modern existential and popular thought. The colloquial "I did the lesser of two evils" is an expression of it. In fact, it can be called the lesser-evil view.
Although there are Christian roots for conflicting absolutism, they grow in Greek soil. The ancient Greek tragedies often portrayed lesser evil situations. In the fifth century B.C., Sophocles and Euripides wrote dramas about heroes who contended against forces of fate they could not avoid. These dramatized dilemmas reflected the nature of the real world of moral conflicts with which conflicting absolutism struggles.
The concept of lesser evils was given a new dimension with the Reformation doctrine of depravity, particularly as that was developed by Martin Luther. There are two things imbedded in Lutheran thought which give rise to a form of conflicting absolutism, the first being Luther's theory of two kingdoms. He believed that Christians live simultaneously in two kingdoms, the kingdom of God and the kingdom of this world. Since they are opposed and since Christians have responsibility in both, it is inevitable that there will be conflicts.
...Perhaps the most comprehensive contemporary exposition of the conflicting absolutist viw is found in Thielicke’s works. There are several elements in Thielicke's form of conflicting absolutism. Fundamental to his view is the belief in real, unavoidable moral conflicts, for "to deny the conflict situation is to deny decision." In conflict situations, says Thielicke, "I may have to face the possibility that what is involved here is a borderline situation which does not allow of any easy solution." And "I can reach such a decision only by going through the conflict and enduring it, not by evading it in the name of some kind of perfectionism. In the conflict situation sin is unavoidable, for we "constantly fall into sin in the borderline situation." In view of human depravity, these kinds of conflicts should be expected, since "the form of this world is no more able to produce absolute righteousness than our human heart." The consequence is that in this fallen world "conduct is de facto a compromise between the divine requirement and what is permitted by the form of this world.... by the manifold conflicts of duty. Even the so-called just war unavoidably involves injustices. For "there is no such thing as a wholly just war, and my decision to endorse a given war and participate in it can be made only from the standpoint that I see, or think I see, greater wrong on the one side than the other..."
Moral depravity is the cause of moral dilemmas. A moral conflict is “not due to the character imparted to the world by creation, as though 'from the beginning' (Matt. 19:8). . . ." No, "it is due rather to the complex of wrong decisions which lie behind us, which have their ultimate root in that primal decision recorded in the story of the fall." In brief, moral conflicts arise out of the fact that this is a fallen world. In such a world, there will be times when we cannot avoid evil. When decisions are made in conflict situations, we must choose the lesser evil, for "there are heavier sins and lighter sins." They are both sins, but "they do not have the same weight." Thielicke makes it clear that there is no justification of doing the lesser evil. Neither is there any pragmatic justification. For “the slogan’ to prevent something worse’ is always ethically destructive because it subjugates our action to a non-Christian pragmatism. In fact, “readiness to do wrong in order to ‘prevent something worse’ is a very dubious principle, because it implies that the end justifies the means...” We must simply recognize that in conflict situations both commands are our moral duty and that sin is inevitable. Nonetheless, since there are lesser and greater sins, the Christian should do the lesser sin, knowing forgiveness is available. “He knows that her in this world there is no perfect righteousness, but he does not therefore draw the conclusion that everything is under the same condemnation and that everything is equally permissible....” On the contrary, he realizes that there is a “quantitative distinction between reprehensible and less reprehensible, between good and less good possibilities.”
According to Thielicke, “we can undergo and endure borderline situations and...inescapable conflicts only under forgiveness.” The Christian knows “that even in a war which– given things as they are– is ‘just,’ [he] must always stand in need of forgiveness.” Thus our “certainty that acts done under the guidance of the Spirit are, despite their ‘crooked’ form, done in God’s name as his affair, and that at the same time they nonetheless stand in need of forgiveness...” Thus in the conflict situation the Christian “acts in the knowledge that even those actions which conform to the ultimate norms perceptible in this aeon must stand under forgiveness...” In short, even our best effort in obeying God’s commands is an evil that needs to be forgiven.
With that in mind, is the goal of preventing conception always sinful? I say yes. But I also believe there are potential situations when contraception might be the lesser evil. But no one has an entirely sinless reason for contraception. This makes the license Roman Catholicism gives for NFP fall on its head. Can abstinence ever be sinless within marriage if it is for the purpose of preventing a life from being created by a man and wife? 1 Corinthians 7:5 tells us that couples may abstain for a time, but only for the purpose of more devote fasting and prayer, not for the purpose of family planning.
My contention is that any acceptable use of family planning will always be an example of trying to chose the lesser evil. These are rare but difficult decisions that cry out for the aid of one's pastor. But how many pastors understand God's law in the area of procreation? Sadly almost none.
So, let me take this a step further and confess my own sin. Even my best effort at obeying God's command to be fruitful is in need of forgiveness. I do not use any form of contraception [pat self on back - sinful pride], yet I am still guilty of being contraceptive in my heart. We're expecting #7 in another couple months, and it's already hard raising six children. My sinful hope is (and I'm sure will be) that my wife's on-demand nursing will prevent conception for a time. Don't get me wrong, I do LOVE our large family and believe I am very blessed, just as it says in Psalm 127. But I am at the same time saint and sinner, just as Paul confesses in Romans 7. I know I should be open to God's will at all times, even when it seems inconvenient to me.
My knowledge of God's law regarding procreation curbs me from utilizing overt methods of family planing through the first use of the law. And it instructs me in loving God and His immutable good will for us via the third use of the law. But it also on this earth will always convict me via the second use of the law. Even my best good works stand in need of forgiveness. I sometimes wonder if I should even get out of bed in the morning, because I know it will lead to more sin. But it is a greater evil to stay in bed and not fulfill my vocations as much as humanly possible, so I get up and accept the sad reality that I will keep sinning.
"What a wretched man I am. Who will save me from this body of death? Thanks be to God - through Jesus Christ our Lord!" [Romans 7:24-25]