Some interesting ideas here concerning Natural Law and since it's been brought up let me try to bring up some relevant points.
To begin, Natural Law is a philosophy which takes the position that rights (since law is concerned with rights) and values (since the Church is concerned with morality) are implicit by nature—in a sense, inherent—which can be distilled through the use of reason (in other words, if it's not rational it's not natural law). In short, this historical model of natural law attempts to use reason to determine what is part of human nature and then use that knowledge to discern morality. I wish to make a point here of stating that this law of nature was to be determined by nature.
One important distinction that I need to make right off is that natural law and divine law are not the same. Divine law proceeds from God and natural law proceeds from nature. God created nature and did so, giving it an essential essence—an essential nature, if you will—that is good and proceeds from the divine while not being, in itself, divine. Goodness, of course, is a divine quality but does not necessarily proceed from the Holy. For example: Aquinas taught that morality has two components to it—correct action and correct motive. Performing an act of charity to impress someone is not a moral action; behaving well because you're in a crowd and are afraid to appear foolish is also immoral. Likewise, performing an immoral act for a good motive is also wrong.
Eternal law is yet another distinction whose delineation is important for a reason I'll get to in a moment. Eternal law is understood to be the binding law that God has set out for the whole universe; and humanity's participation in eternal law is what we finally come to understand as natural law and, according to Aquinas, is to be uncovered through the use of reason. To quote the Saint, "All the inclinations of any parts whatsoever of human nature, e.g., of the concupiscible and irascible parts, in so far as they are ruled by reason, belong to the natural law, and are reduced to one first precept, as stated above: so that the precepts of the natural law are many in themselves, but are based on one common foundation."
It was also Aquinas' understanding that all human tendencies are geared toward real human goods. In this context, my ability to fall in love with only men is a tendency which is at base good because it is geared toward what might be understood as the actual good of what we call romantic love—having a wife and children. Aquinas, in this matter, seems to have understood the word love to mean "to will the good of another."
In order to know what is right, you have to use reason and apply it to Aquinas' precepts.
Aquinas explains that "there belongs to the natural law, first, certain most general precepts, that are known to all; and secondly, certain secondary and more detailed precepts, which are, as it were, conclusions following closely from first principles. As to those general principles, the natural law, in the abstract, can nowise be blotted out from men's hearts. But it is blotted out in the case of a particular action, insofar as reason is hindered from applying the general principle to a particular point of practice, on account of concupiscence or some other passion, as stated above (77, 2). But as to the other, i.e., the secondary precepts, the natural law can be blotted out from the human heart, either by evil persuasions, just as in speculative matters errors occur in respect of necessary conclusions; or by vicious customs and corrupt habits, as among some men, theft, and even unnatural vices, as the Apostle states (Rm. i), were not esteemed sinful."
So primary precepts cannot be blotted out but the secondary ones can be.
In the case of homosexuality, it has been said that the act is intrinsically disordered. The follow up question is, On the basis of what? The response often given is that homosexual acts are not ordered toward procreation to which the response might be, Neither are those acts between infertile couples. At this juncture, the goal posts are usually shifted and the response is usually to say, But the union must be OPEN to procreation. To which a respondent might ask, Why?
Why is it imperative that a union be open to procreation? Why is the use of sex for pleasure—though, let's be honest: sex between loving and committed couples (even infertile ones) is never "just for pleasure" since there's obviously an expression of devout affection in the act—wrong?
Because sex is meant to be ordered toward procreation, is often the answer.
The response here is to ask, Then why do we feel pleasure? We feel pleasure, and this is part in parcel with Catholic teaching, because it has the effect of directing the human being toward that which is good!
Other writers such as St. Augustine (bless his soul—I love him, but I think he was dead wrong in at least this one particular) posited that sex in the Garden of Eden was initially without pleasure. His detractors came back and said, Well then how did Adam and Eve have sex (according to God's command, Be fruitful and multiply) if before the Fall there were no pleasure? More pointedly, they asked how Adam could have even achieved an erection without pleasure? Augustine's response was, and I'm not joking here, "By sheer willpower!"
The argument is, as far as I'm concerned, dead in the water. The idea that sex should be pleasureless or that seeking pleasure is an intrinsic evil is ridiculous. It can be directed toward evil, sure, in that it can lead you to seek out material goods rather than spiritual ones; however, there is little room to see sex between a loving couple—even a gay couple—as disordered on the basis of its not-being-open-to-life. There's much more to sex than that and much more to human relationships.
I also have some problem with the term homosexual acts because often times that's left open—though we generally take that to mean physical acts of a sexual nature. If I had a partner and were married to a man, would that be a homosexual act? Technically, yes--it is the act of getting married. How about the act of loving? Forget sexual acts, a caress or a kiss could be, to some, perceived as a mortal sin that would send me straight to Hell.
So suffice it to say that I see enough problems not necessarily with natural law but certainly with its application; and many in the Church desperate to toe the party line will use natural law to mean whatever they want it to mean--in this case to condemn people.
