The two requirements of sexual intercourse within marriage are:
- Procreation (to make babies)
- Unity (union with spouse)
On the first, the Church teaches that the act must be "ordered to" procreate. Meaning that there ought to be no artificial impediment to the act of procreating. They also say that the act should be "open to life"
On the second point, the couple should have physical intimacy. (This rules out things like artificial insemination as misuse of sexuality.)
The Church has authorized a method called NFP (Natural Family Planning) which basically means you can time having sex so that you don't have kids. However, even NFP can't just be used in any circumstance because you don't want to have kids, as noted in the Catechism:
2368 A particular aspect of this responsibility concerns the regulation of procreation. For just reasons, spouses may wish to space the births of their children. It is their duty to make certain that their desire is not motivated by selfishness but is in conformity with the generosity appropriate to responsible parenthood. Moreover, they should conform their behavior to the objective criteria of morality:
When it is a question of harmonizing married love with the responsible transmission of life, the morality of the behavior does not depend on sincere intention and evaluation of motives alone; but it must be determined by objective criteria, criteria drawn from the nature of the person and his acts criteria that respect the total meaning of mutual self-giving and human procreation in the context of true love; this is possible only if the virtue of married chastity is practiced with sincerity of heart.
Thus given the context for your question was a discussion about people in poverty, the Cardinal was talking about people in need being able to control their family size through NFP. Perhaps there is a deeper philosophical point he's trying to make that the Church doesn't categorically just say "no birth control", but NFP is really the only method we know of that meets the criteria.
The above is perhaps better explained by Humanae Vitae (Pope Paul VI, 1968). Note that it talks about:
- Not using birth control to prevent conception
- That these inanimate objects are not evil in themselves (i.e. a woman could use what is essentially a birth control pill if the objective of that pill is therapeutic for a reason other than preventing birth)
- That you can space out births by timing sex.
Unlawful Birth Control Methods
- Therefore We base Our words on the first principles of a human and Christian doctrine of marriage when We are obliged once more to
declare that the direct interruption of the generative process already
begun and, above all, all direct abortion, even for therapeutic
reasons, are to be absolutely excluded as lawful means of regulating
the number of children. (14) Equally to be condemned, as the
magisterium of the Church has affirmed on many occasions, is direct
sterilization, whether of the man or of the woman, whether permanent
or temporary. (15)
Similarly excluded is any action which either before, at the moment
of, or after sexual intercourse, is specifically intended to prevent
procreation—whether as an end or as a means. (16)
Neither is it valid to argue, as a justification for sexual
intercourse which is deliberately contraceptive, that a lesser evil is
to be preferred to a greater one, or that such intercourse would merge
with procreative acts of past and future to form a single entity, and
so be qualified by exactly the same moral goodness as these. Though it
is true that sometimes it is lawful to tolerate a lesser moral evil in
order to avoid a greater evil or in order to promote a greater good,"
it is never lawful, even for the gravest reasons, to do evil that good
may come of it (18)—in other words, to intend directly something which
of its very nature contradicts the moral order, and which must
therefore be judged unworthy of man, even though the intention is to
protect or promote the welfare of an individual, of a family or of
society in general. Consequently, it is a serious error to think that
a whole married life of otherwise normal relations can justify sexual
intercourse which is deliberately contraceptive and so intrinsically
wrong.
Lawful Therapeutic Means
- On the other hand, the Church does not consider at all illicit the use of those therapeutic means necessary to cure bodily diseases, even
if a foreseeable impediment to procreation should result there
from—provided such impediment is not directly intended for any motive
whatsoever. (19)
Recourse to Infertile Periods
- Now as We noted earlier (no. 3), some people today raise the objection against this particular doctrine of the Church concerning
the moral laws governing marriage, that human intelligence has both
the right and responsibility to control those forces of irrational
nature which come within its ambit and to direct them toward ends
beneficial to man. Others ask on the same point whether it is not
reasonable in so many cases to use artificial birth control if by so
doing the harmony and peace of a family are better served and more
suitable conditions are provided for the education of children already
born. To this question We must give a clear reply. The Church is the
first to praise and commend the application of human intelligence to
an activity in which a rational creature such as man is so closely
associated with his Creator. But she affirms that this must be done
within the limits of the order of reality established by God.
If therefore there are well-grounded reasons for spacing births,
arising from the physical or psychological condition of husband or
wife, or from external circumstances, the Church teaches that married
people may then take advantage of the natural cycles immanent in the
reproductive system and engage in marital intercourse only during
those times that are infertile, thus controlling birth in a way which
does not in the least offend the moral principles which We have just
explained. (20)
Neither the Church nor her doctrine is inconsistent when she considers
it lawful for married people to take advantage of the infertile period
but condemns as always unlawful the use of means which directly
prevent conception, even when the reasons given for the later practice
may appear to be upright and serious. In reality, these two cases are
completely different. In the former the married couple rightly use a
faculty provided them by nature. In the later they obstruct the
natural development of the generative process. It cannot be denied
that in each case the married couple, for acceptable reasons, are both
perfectly clear in their intention to avoid children and wish to make
sure that none will result. But it is equally true that it is
exclusively in the former case that husband and wife are ready to
abstain from intercourse during the fertile period as often as for
reasonable motives the birth of another child is not desirable. And
when the infertile period recurs, they use their married intimacy to
express their mutual love and safeguard their fidelity toward one
another. In doing this they certainly give proof of a true and
authentic love.
(the full document: http://www.vatican.va/holy_father/paul_vi/encyclicals/documents/hf_p-vi_enc_25071968_humanae-vitae_en.html)
