A - Thanks for the question.
I have written about the basics of contraception many times before. The most recent was here.
But, before we go further, we need to see the verse in the Catechism that you are referring to. It says (in context):
Fecundity is a gift, an end of marriage, for conjugal love naturally tends to be fruitful. A child does not come from outside as something added on to the mutual love of the spouses, but springs from the very heart of that mutual giving, as its fruit and fulfillment. So the Church, which is "on the side of life" teaches that "it is necessary that each and every marriage act remain ordered per se to the procreation of human life." "This particular doctrine, expounded on numerous occasions by the Magisterium, is based on the inseparable connection, established by God, which man on his own initiative may not break, between the unitive significance and the procreative significance which are both inherent to the marriage act."Here the Catechism is quoting Humanae Vitae. Which says in 11:
The Church, nevertheless, in urging men to the observance of the precepts of the natural law, which it interprets by its constant doctrine, teaches that each and every marital act must of necessity retain its intrinsic relationship to the procreation of human life.It explains this teaching further on in the document in 12:
This particular doctrine, often expounded by the magisterium of the Church, is based on the inseparable connection, established by God, which man on his own initiative may not break, between the unitive significance and the procreative significance which are both inherent to the marriage act.In other words, natural infertility is not the same as using contraception. Some are naturally infertile because of health issues or age. This is not consciously denying the gift of infertility. Notice the wording above that says "on his own initiative", which refers to using contraception to make oneself infertile and thus consciously separate fertility and the sexual act - which then intentionally divorces sex from one of it's purposes.
In the Code of Canon Law is states:
§3. Sterility neither prohibits nor nullifies marriage, without prejudice to the prescript of can. 1098.A Christian marriage does not depend upon fertility. But, Christian spouses cannot act contrary to the moral law just because they are no longer fertile or never were. This means that all acts that are sinful for a fertile couple are still sinful for an infertile couple.
Hopefully this helps.