I would reiterate my belief that the Catholic Church has the right to teach anything, as does any other religion, and that the Church also has the right to demand obedience, period. At least in the Western world, people are free to join, or leave, any denomination or any faith. The fact that so many Catholics choose to dissent from the Church's teaching on contraception doesn't change the right of the Church to demand their obedience.
One of the things that engages me about this particular question is that, in my experience, those who support the Church's teaching generally start their defense not with a call to obedience, but with a call to the teaching's supposedly self-evident truth. It is only when that line of reasoning fails that the Church's authourity is brought up, almost, one might say, as a "trump card". And the "self-evident truth" part is what I question. If it really is self-evident, why should any appeal to authourity be required?
It's difficult to be brief, but I'll try to do so in outlining my thought processes.
1) The Catholic Church defines something known as "mortal sin". Most of these sins reference behaviours that have been condemned in pretty much every society of which we have records: killing, stealing, adultery are several that come to mind. One doesn't need to be Catholic, or Christian, to understand why these behaviours are problematic and harm society. Taking it to an extreme, one can be a complete atheist and still agree that these things are wrong, even if one doesn't accept the use of the actual word "sin".
2) At the same time, the Church also teaches that it is a mortal sin to miss Mass on Sundays and Holy Days of Obligation. This teaching is not rooted in universal behaviours - it is specific to the Church and members are expected to obey. I am sure there are theological underpinnings (I've never really investigated), but for my point it's not necessary to understand what they might be. Catholics are expected to attend Mass, period.
3) The Church recognises that individual circumstances sometimes mitigate the mortal sinfulness of a given action. Killing in self defense is not murder. Stealing from someone who hoards all available food is not necessarily stealing in a sinful sense. The obligation to attend Mass is suspended in circumstances of illness, age, dangerous weather conditions, general infirmity and probably some other conditions of which I am unaware. The main point is that the action taken, in and of itself, is not a mortal sin if mitigating circumstances apply.
And then I look at the teaching on contraception, and the argument that the use of any "artificial" means to avoid conception is "intrinsically evil" and a mortal sin, regardless of pretty much any mitigating circumstances one can think of, and I try to find the self-evident, underlying, universal reason why this is so. In other words, I try to find a reason that does not require a belief in God. Because, it seems to me, if something is that terrible ("intrinsically evil" is a pretty strong statement), the reasons why should be obvious across all belief systems and societal structures. They should be convincing to an atheist. And THAT is the argument that I have yet to find.
For example, contraception is often linked with abortion, as though they are two sides of a single evil coin. And yet there are many people who argue against abortion based on the simple belief that once an egg and sperm unite, a new life begins and abortion is murder. This argument is made with no reference to God or ensoulment or any other religious concept - it simply stands on the objective facts. Unlike, in my experience so far, the arguments against contraception.
So, ultimately I have a great deal of trouble fitting the Church's teaching on contraception into my understanding of the Catholic framework. The use of contraception is a mortal sin but there is no easily understood, universal, non-theistic rationale for why, as there is for most sins that deal with human behaviour. At the same time, if at heart the teaching is based on obedience, rather than on an objective truth, it's far more onerous than other teachings based on obedience.
Please note that to this point I am not actually arguing against the Church's teaching as much as noting where its general outline doesn't make sense to me. I'm not sure if that's at all "debate worthy".
However, personally, I have yet to hear an attempt to argue, objectively, against the use of contraception that doesn't ultimately break down and resort to obedience, and therefore remains unconvincing. Still less have I encountered good arguments to defend the Church's internal inconsistencies with respect to this teaching. Doubtless my own experiences colour my reactions. But I don't think it's appropriate to go into a detailed analysis of all that, or at least not right now. That would involve many pages for perhaps no good reason. Although I am open to further dialogue should you wish to reply.
Maggie, I'm honored that you took up my invitation so of course I wish to reply.
First, I'm rather surprised you have found people who claim that the Church's teaching on contraception ('CTC' for short) is self-evidently true. Self-evident truths are those like '2 +2=4' and 'Good is to be done and evil avoided'; but if CTC is true, its truth is not evident in that sort of way, nor does the Church say it is. People who say otherwise either misunderstand what 'self-evident' means or are just plain wrong. On the whole question of reason and morality, I suggest you read J. Budziszewski's What We Can't Not Know.
Perhaps all such people mean is that the teaching is reasonable inasmuch as one can adduce rational grounds for it other than appeal to authority. In his encyclical Humanae Vitae (esp. §17 ff), Pope Paul VI himself does just that. His predictions about the social consequences of widespread contraception have proven to be chillingly accurate, which is as good evidence as any that CTC is true, if that's the sort of evidence one requires. But before we can fruitfully discuss the various grounds for CTC further, we need to be clear about what that teaching is and what the grounds for it could be in principle. I don't think you're quite there yet.
I say that because I think you're really mischaracterizing what is at issue. You say: "I look at the teaching on contraception, and the argument that the use of any "artificial" means to avoid conception is "intrinsically evil" and a mortal sin, regardless of pretty much any mitigating circumstances one can think of, and I try to find the self-evident, underlying, universal reason why this is so. In other words, I try to find a reason that does not require a belief in God. Because, it seems to me, if something is that terrible ("intrinsically evil" is a pretty strong statement), the reasons why should be obvious across all belief systems and societal structures. They should be convincing to an atheist. And THAT is the argument that I have yet to find." Well, you're not going to find any such argument, because the claim for which you think there needs to be such an argument is not CTC.
In moral theology, to say that such-and-such is "intrinsically evil" means that it's the sort of act that is objectively wrong irrespective of motive or circumstance. However, it does not follow that the agent is always subjectively culpable for doing it. There are factors that can and do diminish an agent's moral responsibility and thus their culpability. Given as much, an act is a "mortal sin" if, in addition to being intrinsically wrong, the agent knows it's wrong and does it freely, without inner or outer compulsion. (There's also the case where the agent is culpable even if they didn't "know" the act was wrong because there is no excuse for their having been ignorant. But that brings us to murky waters we need't navigate here.) But all that is different from what you seem to mean by 'mitigating circumstances'. By that you seem to mean 'non-standard circumstances', or something like that. But citing such circumstances only enables us to determine what sort of act in question; thus, two acts of killing that are physically identical can be morally different depending on the intentions of the agent and the victim. But all that tells us is that intent makes a difference to determining what sort of act is in question; it doesn't help us identify which sorts of act, if any, are always "intrinsically evil" and which are not. That depends on other considerations.
Now in the case of CTC, the Church says that contraception is intrinsically wrong. That does not mean that anybody who goes in for it commits a mortal sin; for many such people either don't know it's wrong or are party to it only involuntarily. It does mean, however, that if one directly interrupts the generative process at any stage, one is doing something objectively wrong.
For that claim, you seem to want an argument that makes no reference whatever to God. I don't think that's a reasonable requirement. Here's why.
First, and as a reading of Humanae Vitae shows, CTC is of a piece with, indeed logically depends on, Church teaching about marriage. The latter includes the claim that marriage and its nature were instituted by God. We did not invent it; rather, the institution of marriage is something given that we more or less conform ourselves to. Knowing as much doesn't necessarily require any appeal to divine revelation; it is discoverable by reason as a precept of the natural law. But that doesn't mean God should or even can be left out of the picture. The U.S. Declaration of Independence, for example, makes reference to "the laws of nature and of nature's God." Some signatories of that document were deists, not Christians; some of the Christians did not believe that the Bible is, in toto, a true record of divine revelation. But they all agreed that the moral law, whatever its content, requires a transcendent source, a Higher Power as lawgiver, in order to have the force of prescription rather than just that of description. Call that "theism" if you like; but it is wholly in keeping with what most people of every culture have believed and continue to believe. The notion that either the content or the practice of morality should be ultimately sustainable without God is just one philosophical view among others. It is not my view, it is not the consensus of the human race, and it is certainly not a teaching of the Church.
Second, the epistemic status of CTC is really no different from that of abortion. The Church's teaching against abortion is easier for some people to understand and accept than is CTC because, as you correctly note, one doesn't need theistic premises in order to establish that the embryo is a human person. And if one also assumes that it's intrinsically evil to kill innocent human persons directly and voluntarily, then one has got an argument that abortion is an instance of such killing and is therefore intrinsically evil. But whence comes the assumption? It isn't evident to everybody, let alone self-evident; some women will admit that fetus is a human being but still think the quality of the woman's life trumps continuation of the fetus's. And in World War II, most Christians thought it perfectly acceptable for the Allies to bomb civilian targets for strategic purposes, thus killing countless innocent people.
True, the Church condemns such actions; in his encyclical Evangelium Vitae (§57 ff), John Paul the Great even says:
[b]y the authority which Christ conferred upon Peter and his Successors, and in
communion with the Bishops of the Catholic Church, I confirm that the direct and
voluntary killing of an innocent human being is always gravely immoral. This doctrine, based upon that unwritten law which man, in the light of reason, finds
in his own heart (cf. Rom 2:14-15), is reaffirmed by Sacred Scripture, transmitted by the Tradition of the Church and taught by the ordinary and universal magisterium. (Emphasis added.)
But what does that tell us? Let's face it: we live in a fallen world, in which human beings can too easily dull their consciences by rationalizations based on expediency. So, what's based on "the unwritten law" accessible to human reason just does need constant clarification and reinforcement by divinely instituted authority. And that's all that Paul VI did in Humanae Vitae with CTC.
You do make reference to the "internal inconsistencies" in CTC, but at this point I'm not sure what you think they are. I believe I already rebutted the most plausible version of that claim in my above-referenced Pontifications post. So if you wish to reply, please focus on that.