Andrea has promised to come back and argue in favor of natural law, but it occurs to me that the comment thread we were on has gotten very long and unwieldy, so I thought I'd write a new post to continue the discussion. I'll include a more complete explanation of the problems I see in "natural law" arguments against, for instance, birth control and homosexuality.
First, I have to clarify, because "natural law" is not a clearly defined term. When some people say it, they simply mean "those ethical rules we can derive through reason alone," or "the moral intuitions everyone agrees on." Of course I have no objection at all to this idea. I've written a number of things on the emotional and rational grounding of morality. Simple rules of thumb for moral action include things like, "Would I like it if someone did this to me?" "Does this action harm anyone, either inherently or through some distant side effect?" or "Would I like to live in a world where everyone acted like this?" These rules are reasonable -- I can demonstrate that it is to everyone's benefit if we all follow them -- and they are universally applicable.
The sort of "natural law" talked about in these arguments, though, is much more specific. The natural-law argument that underlies Catholic sexual ethics is as follows: Human faculties possess a teleology, a purpose or end. For instance, the sexual faculty exists to create children. The pleasure we get from using it is meant as a spur to get us to beget children, and to seek out the pleasure while frustrating the primary end, reproduction, is a perversion of the sexual faculty.
The first simple objection I can make to this is that teleology implies a creator. That is, nothing has an end unless someone creates it for that end. If I create a car, a fork, or a chair, I create them the way I do because I have a purpose in mind for that thing. To comb my hair with a fork or to stand on a chair would be misusing that thing, and if it's my item you're misusing, I might object. However, from a human perspective there is no "proper use" of a tree. It simply is, because we didn't make it for any particular purpose. We can turn it to any purpose we like -- we can make it into chairs or into paper or we can sit underneath it. It might be better for some of those purposes than others, but no one will object to how we use it if it's our own tree.
If God created us, it's reasonable to say he built in our different functions and desires for his own purposes, and he might have an opinion about how we use them. If, however, we came into being through the blind process of evolution, nobody actually cares what we do besides other humans. Evolution is a terrible source for morality, because it doesn't care about human concerns like happiness, love, or beauty. If we took evolution as a guide, we should probably give up things like art and music, each of us should have the maximum number of children we possibly can -- celibacy or late marriage would be sinful -- and we should euthanize our grandparents because they're a waste of resources. That, need I point out, would be terrible, so I vote for "kick evolution in the face and do things it never intended."
So if you are already Catholic, it is quite reasonable for you to believe in natural law. However, the church claims that natural law is available to everyone, regardless of their religion, through the unaided light of reason alone, and it just does not appear that this is true. You have to have a prior belief in God, and it's pretty evident that reason alone does not bring everyone to a belief in God -- particularly not a specific version of God that has specific expectations for what we are to do with our bodies.
The second problem with natural law is that it doesn't really distinguish between things that are perversions of a bodily function and things that aren't intended by our body but are just fine. For instance, if the purpose of the sexual faculty is to beget children, you would think that we are all bound to beget children since we all have a reproductive system, but the church says no to that -- you can forego using the sexual faculty altogether, provided you don't use it in a way that frustrates procreation. That seems a distinction without a difference -- as though the point was to make sure those who don't have children suffer for it, rather than to get everyone to have children.
And, while natural-law proponents make a huge deal over the difference between sex between naturally infertile people and artificially infertile people, it does not appear to me that there is a real difference. In both cases, the effects are the same; and the intentions of the people may be the same too. (For instance, a couple that only has intercourse when one of them is infertile is deliberately avoiding procreation.) Only the means varies, and what is the justification for making the means matter? What is intrinsically bad about the means?
Another issue, when you bring homosexuality into the equation, is that a gay couple is not deliberately frustrating procreation. They might want to procreate, like an infertile couple might, they just can't. Like the infertile couple, they experience the other ends of sex, like pleasure and bonding. Like the infertile couple, they don't experience the procreative end, but that's not intentional. It's a massive stretch to me to imagine that there is a teleology of the body that exists to encourage procreation, but it cares about things irrelevant to whether a couple procreates.
And then, of course, we can point out that sex is the only faculty of the body that is treated this way. The digestive system can be frustrated in similar ways, but no one makes a fuss about it. You can drink a zero-calorie soda. You can chew gum, which gets your stomach growling as it expects food, but which will never nourish you. You can do all this purely to get the pleasure of food while not nourishing your body -- even though the only reason it is pleasurable to eat is to get you to nourish your body. That doesn't matter; no one thinks this is immoral. And if you're sick, it's okay to nourish the body without eating -- with a feeding tube or an IV. That's perfectly fine, even though begetting children without sex is not okay. The rules for sex are different, and I don't think there is a rationally-explicable reason why this should be.
It's a general medical principle -- accepted by the Church -- that you can damage one part of the body to save the whole. You can take out a diseased kidney. You can irradiate the body to kill a tumor even if it makes the rest of you sick. You can do a gastric bypass which hinders the digestive system in order to help a person lose weight. All of this is okay! But sterilizing a person because pregnancy will risk their life is not okay. I can't see a clear rule which you could figure out ahead of time that would allow you to draw these conclusions. Instead, it seems like people had the conclusions in their minds in advance and tried to come up with an argument that would justify them.
And you know what? That's totally fine. It's okay to say, "We think God created us with a purpose, and through revelation, we know a lot about what that purpose is. Let's spend some time and see if we can figure out what the logical rules are that made God draw the lines where he did." That's a part of religion, but it's not a bad thing. The bad thing is when you claim that non-Catholics could come to the same conclusions without first knowing the rules, and then claim that this means they are bound to your rules despite not sharing your beliefs.
We all know that virtually no non-Catholics ever are convinced by natural-law arguments. The few who are generally wind up converting to Catholicism, because apparently they already believed many of the same first principles. I think it's uncharitable the way that some Catholics put this down to self-deception or outright lying -- that is, "you would be convinced by this argument if you weren't so selfish/lying about not being convinced/morally twisted by your sinfulness." Is it so hard to imagine that people might simply disagree? I myself was ready and willing to be convinced by natural-law arguments in college, and I studied them quite a bit, but I simply felt that without the basic premise that God wanted certain things of us and had revealed what they were, the arguments would never be convincing.
For more on this topic, I recommend this post by Melinda Selmys, as well as the comments. I also agreed with many of the counterarguments in the comments on this post.