Corki,
I have a hard time wrapping my head around this notion. Let’s look at the Catechism:
2351 Lust is disordered desire for or inordinate enjoyment of sexual pleasure. Sexual pleasure is morally disordered when sought for itself, isolated from its procreative and unitive purposes.
2366 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.”
So… separating sexual pleasure aside from the unitive and procreative purposes to which sex is ordered, is morally disordered. Full stop – no discussion of marriage.
Do sexual acts within marriage fall within this discussion? Naturally, they do! However, the catechism doesn’t seem to discuss this as an “either/or” thing, but as a “both/and” thing: sex outside of its procreative end is wrong both per se and considered in the context of marriage.
I can see how it sounds weird… it makes us consider the degree of sinfulness of an act that is a priori sinful. Nevertheless, the Church doesn’t flinch: all sexual acts must be open to procreation.