To utilitarian Americans, this seems like a pointless distinction: The end justifies the means. When we want to lose weight, we get liposuction. When we want the kids to sit still, we give them Ritalin. When we want more milk from a cow, we genetically engineer it using alien DNA from a grasshopper. Descartes taught us long ago that the point of science was to become the "master and possessor of nature." But the Church sees Creation as covered with big, greasy divine fingerprints, which we're not supposed to wipe away in our rage to tidy things up. And because sexuality is even more sacred than eating, we must treat it with more reverence than we do, say veal cattle. The best way to explain the Church's official theology is to compare spacing children to losing weight. You can achieve that through dieting—or you can try bulimia. If you think they're equivalent, you probably better go back to your gastroenterologist.An ethic based on “If it's not natural, it's bad.” can easily become environmentalist and then Mathusian. (If “natural family planning” becomes popular, we can expect the Other Side to redefine it to mean planning for a world population that can be supported without “frankenfoods.”) Frankenfoods and other equally artificial means will probably be needed to support large populations (as required by God's instructions to Noah).
Come to think of it, the authors seem a little too ready to assume that natural family planning is an alternative to large populations. Natural family planning advocates rarely cite quantitative data. It might slow down population growth but won't stop it. (It's also worth noting that Orthodox Judaism also requires periodic abstinence, but in the other direction. Natural family planning and the Orthodox Jewish laws of family purity are incompatible … which goes a long way to explaining the sizes of Hasidic families).
Meanwhile, persons have rights, even when unborn. Foods don't.