In my post on Aquinas on Sexual Sins I promised to return to the issue of the legalistic morality that became dominant in the Catholic church. For St. Thomas Aquinas, law is fundamentally an ordinance of practical reason perceiving the appropriateness of things to an end, and ordering concrete means to that end. A law which is unreasonable, which does not correspond to the exigencies of man's true end, is neither a true law, nor morally binding on man. The lawfulness of law does not arise simply from the will of the lawgiver, but presupposes knowledge of the subject of the law, and what is appropriate for him. This is true not only of human law, but of divine law as well. God's will and knowledge is not caused by created reality, but is the cause of it. Nonetheless God's knowledge of the orderliness in his plan is logically prior to his will to impose that order as natural or divine law.
Nominalism, which does not recognize an intelligible order in real things as the basis for law, attributes law to will alone. In St. Thomas's understanding, murder is forbidden by natural and divine law because murder is incompatible with, contradicts man's true good. In the nominalist understanding, murder is immoral only because God willed to forbid it. This same understanding is reflected today in the all-too-common statements that, e.g., contraception isn't bad, but it's a sin. Sin is seen as something extrinsic to the human action, rooted in God's positive will alone.
In following centuries this nominalistic, legalistic view of law came to dominate Catholic thought. One significant historical reason for this seems to have been the desire for a system that confessors could use to determine the gravity of sins and appropriate penances. The rule-based approach seemed well-adapted for this purpose. Over time, then, moral theology became separated from man's desire for happiness, practice of virtue, asceticism, prayer, etc., and was more or less restricted to obligations, precepts, and sins.
One dangerous consequence of this view is that, when men's extrinsic motivations for obeying God decrease–because, e.g., they don't believe in hell, or doubt whether God would differentiate men's final destinies on the basis of their actions–they have little motivation at all to obey laws in which they don't see much sense, e.g., the law for openness to procreation and against artificial contraception.
Conscience and relativism
This legalistic view of morality pops up in another way as relativism in the modern emphasis on conscience as a guide of one's action, even if the action is objectively bad. Thomas Aquinas explains that the binding force of conscience consists in the fact that it is the concrete judgment that an action is good or bad; thus the need to follow conscience derives from the need to act well, if one is to attain one's end and be truly happy. But when obligation is separated from goodness and happiness, then the binding force of conscience is no longer seen as based on the need to seek man's true good, but is absolutized. The moral value of man's action is seen as simply consisting in his following his conscience, regardless of whether or not what he does is truly good.