Michael Liccione had and interesting post on Sacramentum Vitae last week about contraception and Catholic doctrine. One important point I thought he touched on is that the Church's teaching on artificial birth control is not 'self evidently' true.
Clearly, (as Michael points out) what someone who claims that the Church's teaching on contraception is 'self evidently' true means is not 'self evident' in the formal sense but rather that the wrongness of contraception may be determined from natural law principles independently of revelation. However, there is a difficulty with natural law which is perhaps not often enough recognized.
When we say that natural law tells us that contraception is wrong, we mean that it can clearly be seen without the benefit of revelation that contraception fundamentally changes the nature of a body act. It takes an act which is, in biological 'design', a source of both fertility and pleasure, and removes the element of fertility (the element which, from a biological point of view, is absolutely essential to the nature of the act.)
However, what cannot be demonstrated without revelation (or some other leap of faith or intuition) is why one should not alter this basic biological meaning. Several non-Catholics have come back at the 'contravenes nature' argument by saying: It's naturally painful to give birth. Pushing something that size out an opening that size will naturally be incredibly painful. Is it, therefore, immoral to give a laboring woman pain killers? Come to that, is it immoral to ever provide pain killers, since pain is a natural biological reaction to certain stimuli? Is it wrong to use NutraSweet because sweetness is meant to come with calories?
I'm not sure that there can be an completely firm answer to this line of questioning other than revelation. Certainly, there is an intuitive sense that it is somehow better to use the human organism as it was meant to be used, but divorced from a revealed understanding of what humanity is 'meant' to be, I am not sure how one can show definitely that people should not attempt to 'cheat nature' on this or any other topic. However, the fact that there is a very strong intuitive feeling which many people experience (and which leads many of them to attempt birth without pain killers, child spacing without contraception, and cooking without artificial flavors and sweeteners) and which can be backed up by a rational analysis of what humans are and how they work, seems like a hint that there is indeed something to the 'the way things were meant to be'.
Nonetheless, there are those who fundamentally do not 'get it'. In my youth (a wonderfully pretencious phrase for a 27-year-old to use) I was an avid reader and also writer of science fiction. Someone gave me at one point a book which was essentially a guide to the science of planning science fiction universes, with all sorts of useful information about types of stars, what effects a thicker or thinner atmosphere might have, etc. One whole section was on cloning and 'genetic art' with all sorts of speculations about the wonderful things people might one day do: creating perfect athletes, unbelievably beautiful models, performance art humans with strangely configured bodies, etc. One of the things that struck me as incredibly 'off' about that chapter was the eagerness and excitement with which the author addressed the topic. And yet, there are pretty much no compelling science fiction stories in which cloning and genetic engineering are seen as an unmitigated good, or even mostly good. Gattaca, Never Let Me Go, Blade Runner, The Matrix, each of these and many more regard playing what what 'human' is to be dangerous is not downright evil. Even Ghost In The Shell, which accepts the standard elements of the cyberpunk genre as starting points, sees a very real sadness in what is lost in a world where people can download their consciousness' into cybernetic bodies.
But none of that is something one can 'prove' without accepting certain things about humanity: primarily that at some fundamentally level we are 'meant' to be what we are. If one is not willing to take that as a fundamental principle (whether one derives it from revelation, philosophy, or personal preference) then the natural law and language of the body arguments against contraception will fall on deaf ears. At that point, even if the sex is designed to be fertile, why not just change the design? Who says we have to play the cards we were dealt?