The papal encyclical Humanae vitae (HV) teaches that our bodies are not ours to do with as we please:
“In the task of transmitting life, … [married couples] are not free to proceed completely at will, as if they could determine in a wholly autonomous way the honest path to follow; but they must conform their activity to the creative intention of God, expressed in the very nature of marriage and of its acts, and manifested by the constant teaching of the Church.” 10
“[J]ust as man does not have unlimited dominion over his body in general, so also, with particular reason, he has no such dominion over his generative faculties as such, because of their intrinsic ordination towards raising up life, of which God is the principle.” 13
“[T]he Church is the first to praise and recommend the intervention of intelligence in a function which so closely associates the rational creature with his Creator; but she affirms that this must be done with respect for the order established by God.” 16
Thus, regulating births per se is not wrong. What’s wrong is doing it in a way that disrespects “the order established by God”:
“The Church is coherent with herself when she considers recourse to the infecund periods to be licit, while at the same time condemning, as being always illicit, the use of means directly contrary to fecundation, even if such use is inspired by reasons which may appear honest and serious. In reality, there are essential differences between the two cases; in the former, the married couple make legitimate use of a natural disposition; in the latter, they impede the development of natural processes. It is true that, in the one and the other case, the married couple are concordant in the positive will of avoiding children for plausible reasons, seeking the certainty that offspring will not arrive; but it is also true that only in the former case are they able to renounce the use of marriage in the fecund periods when, for just motives, procreation is not desirable, while making use of it during infecund periods to manifest their affection and to safeguard their mutual fidelity. By so doing, they give proof of a truly and integrally honest love.” 16
“[I]f the mission of generating life is not to be exposed to the arbitrary will of men [e.g. controlled by government], one must necessarily recognize insurmountable limits to the possibility of man’s domination over his own body and its functions; limits which no man, whether a private individual or one invested with authority, may licitly surpass. And such limits cannot be determined otherwise than by the respect due to the integrity of the human organism and its functions,” 17
“[The Church] engages man not to abdicate from his own responsibility in order to rely on technical means; by that very fact she defends the dignity of man and wife.” 18
In regard to that last quote, consider the topic of overeating and weight loss: In the not-too-distant future someone may invent a fat-burning pill that actually works, that is, enables you to eat all you want and not gain weight. Now, if someone is concerned about his weight, what method should he use to control it? Should he exercise the discipline required to eat right and exercise? or would it be just as well to take a pill that enables him to indulge his appetite without limit, and throw discipline out the window?
It’s important to understand the Catholic concept of concupiscence, as one of the fruits of the Fall of Adam and Eve. Basically, concupiscence is the tendency of the desires of the flesh to exert themselves in rebellion against the mastery of the intellect and will. In our fallen state, our ability to resist these desires is compromised, and therefore we must battle the flesh constantly. This doesn’t mean that fleshly pleasures are bad per se, but that they’re difficult to keep within proper bounds, and therefore we must exercise moderation and practice regular discipline lest they rage out of control and lead us into sin.
Presumably it is with this in mind that Pope Paul writes,
“The honest practice of regulation of birth demands first of all that husband and wife acquire and possess solid convictions concerning the true values of life and of the family, and that they tend towards securing perfect self-mastery. To dominate instinct by means of one’s reason and free will undoubtedly requires ascetical practices, so that the affective manifestations of conjugal life may observe the correct order, in particular with regard to the observance of periodic continence [abstinence from sex].” 21
So again, it’s fine to regulate the occurrence of conception and birth, but we must do it in such a way “that the affective manifestations of conjugal life may observe the correct order” — that is, we must keep our bodily lusts in check, and not indulge them at will. This is why it’s bad to regulate births by “technical means” (see 18 above), because in relying on such means we are “abdicating our responsibility”. That is, if you can just take a pill to keep from getting fat, then your appetite for food no longer has to be kept under control; you can let it run wild and gorge yourself at will. This may work medically, but is it a good thing spiritually? By the same token, if you can just take a pill to prevent pregnancy, then your appetite for sex no longer has to be kept under control, but is free to indulge itself at will. Is this a good thing spiritually? (Has it been good for our society?)
In summary, our bodies are not our own; we can regulate what our bodies do, but in so doing we must respect the way God made them. Therefore it’s a bad idea to suppress our natural bodily functions for the purpose of enjoying bodily pleasures without consequence; rather, we should be willing to exercise discipline in eschewing bodily pleasures at some times, and enjoying them at other times, with moderation and within proper boundaries, all the while working with our bodies rather than against them, out of respect for God’s design.
[This was adapted from a comment of mine on another blog.]