Why do natural inclinations of human nature give rise to an obligation of natural law?
Is it the mere fact that humans are inclined to this or that good? If so, must one concede the argument in favor of homosexual relationships, that some persons are just naturally inclined to such relationships (granting the premise that it is a natural inclination or at least a natural predisposition triggered by some experiences of one kind or another)?
Or is a natural inclination merely an objective fact, which receives moral value extrinsically, from the purpose imposed on it by human reason. Is a natural law connected with human inclinations only because human reason judges that the good involved in these inclinations (e.g., the good of reproduction, the continuation of the human species in time) is a kind of ultimate good that one cannot reject without in some sense rejecting goodness itself, and offending one's own humanity? In that case, isn't the notion of natural inclination irrelevant? Wouldn't it be just as good, for example, to preserve our lives, and just as bad to commit suicide, even if we didn't have a natural inclination to self-preservation? It is the judgment of reason and the seeking of what is good that is important, not the physical/biological facts.
The International Theological Commission, in its recent document on Universal Ethics and Natural Law holds a mean between these two positions:
79. The rehabilitation of nature and of corporeality in ethics cannot be equated with any kind of "physicalism." Some modern presentations of natural law have seriously denied the necessary integration of natural inclinations in the unity of the person. Neglecting to consider the unity of the human person, they absolutize the natural inclinations of the different "parts" of human nature, approaching them without hierarchizing them, and failing to integrate them in the unity of the entire plan of the subject. [This criticism would (also) apply to the "new natural law" approach taken by Germain Grisez, who enumerates many inclinations (e.g., the inclination to live, to avoid pain, to play, to enjoy aesthetic experiences, to know theoretical truths) which he sees as irreducible, and thus as absolutes–although he sees them as immediate givens of experience, rather than as deduced from the observation of one's inclinations.] Now, John Paul II explains, "natural inclinations do not acquire a moral quality, except insofar as they are connected to the human person and to his authentic realization" (Veritatis splendor, n. 50). Today therefore there is need to hold fast to two truths. On the one hand, the human subject is not a union or juxtaposition of diverse and autonomous natural inclinations, but a substantial and personal whole called to respond to the love of God and to unite himself through a recognized orientation towards a last end, which hierarchizes the partial goods manifested by diverse natural tendencies. Such a unification of natural tendencies in service of the higher ends of the spirit, i.e., such a humanization of the dynamisms inscribed in human nature, does not at all constitute a violence done to it. On the contrary, it is the realization of a promise already inscribed in them.74 For example, the high spiritual value that is manifested in the gift of self in the reciprocal love of spouses is already inscribed in the very nature of the sexual body, which finds in this spiritual realization its ultimate reason for being. [Holding to this one point, we must reject the extreme of "physicalism," which would take the natural inclinations as absolutes that are not subordinate to any higher principle.] On the other hand, in this organic whole, each part preserves a proper and irreducible meaning, ["irreducible" probably means here that the role and value of each part of human nature is not reduced simply to its utilitarian aspect (what does it produce for me?), but that each part of human nature participates in its own way in the human good.] of which reason should take account in the elaboration of the entire plan of the human person. The doctrine of the natural moral law should therefore affirm the central role of reason in the actualization of a properly human plan of life, and at the same time the consistency and the proper meaning of natural pre-rational dynamisms.75 [Holding to this point, we must reject the other extreme, which would take the natural inclinations as mere "matter" for human action, devoid of any intrinsic human teleology. This is explained in further detail in the footnote.]
 The duty to humanize the nature in man is inseparable from the duty to humanize external nature. This justifies the immense effort made by men to emancipate themselves from the coercions of physical nature in the measure in which they hinder properly human values. The struggle against illness, the prevention of hostile natural phenomena, the improvement of the conditions of life are of themselves works that attest to the greatness of man called to fill the earth and to subdue it (cf. Gen 1:28). Cf. Pastoral Constitution Gaudium et spes, n. 57.
 Reacting to the danger of physicalism and rightly insisting on the decisive role of reason in the elaboration of the natural law, some contemporary theories of natural law [e.g., those of Josef Fuchs, Charles Curran, or Richard McCormick] neglect, or rather reject, the moral significance of the natural pre-rational dynamisms. [According to these theories] the natural law would be called "natural" only in reference to reason, which would define the whole nature of man. To obey the natural law would be therefore reduced to acting in a rational manner, i.e., to applying to the totality of behaviors a univocal ideal of rationality generated by practical reason alone. This means wrongly identifying the rationality of the natural law with the rationality of reason alone, without taking account of the rationality inherent in nature. [An example in the text of such inherent rationality is the "gift of self" that is "already inscribed in the very nature of the sexual body."]
[Paraphrase/exposition of the note: Reacting to the one extreme, some theories see the "lawfulness" of the natural l-aw as entirely constituted by human reason, rather than as recognized and ordered by human reason, yet originally constituted by God's "reason". Practical reason does not perceive any moral value and signification in natural inclinations, but only "practical" value. While recognizing the truth that law is a work of reason, this position overlooks the fact that the natural law in us is a participation in the eternal law, which is a work of God's reason. This participation is found both in that which is essentially rational–the reason–and in that which reason by participation–the inclinations, which participate in God's plan as being directed by it.]
In accordance with the one truth, that reason exercises a discernment in regard to natural inclinations, we must recognize the possibility that reason discerns in a particular case that a natural inclination does not represent such "inherent rationality," but is rather contrary to reason. The affirmation that the inclination of nature is a participation in and expression of God's plan does not mean that every particular inclination of every particular nature is such. According to Thomas Aquinas, some individuals have a natural inclination to particular sins, on account of a corruption of nature, (ST I-II, 78:3) and he also, following Aristotle, states that something contrary to the human species may become per accidens natural to an individual, (ST I-II, 31:7) on account of a corruption of some natural principle–as evidenced in a connatural desire to eat dirt or coal or other human beings, or to have bestial or homosexual intercourse. Indeed, one might argue that there are very frequently present in men some natural inclinations that exceed the bounds of reason, as an inclination for a man to kill an adulterous wife, or (inordinate in a context where food is always plentiful) to eat a great deal whenever plenty of good food is available.
In accordance with the other truth, the goods and potential evils involved in the use of various human faculties is sufficiently determined by human nature itself, and sufficiently luminous to be not only a locus, but also a source of moral insight. Although the actual moral value of any particular action depends on our reason and will (what we do involuntarily or without appreciation of what we're doing may be good or bad, but not morally good or bad in the full sense of the term), "What we're doing" when we make use of those human faculties isn't so much imposed by us, as recognized by us.