I did not say that cannot be the intent. In fact, the Catholic goal is to preserve the procreative
and the unitive aspect.
The couple who engages in the act during a naturally infertile period must be conscious of their own intent for doing so -- are they doing so to avoid having children for selfish purposes, being fully able to provide for a new child yet not wanting to? Or are they sincerely wanting to have a child yet taking into account their current circumstances which would not allow the child to have his/her basic necessities?
In either case, they are avoiding pregnancy -- regulating birth -- but in one case, the couple shuns the possibility of life while in the other, the couple embraces potential new life with an honest concern for the well-being of offspring.
If you're using condoms to prevent the spread of STDs, especially if you have the STD, you shouldn't be having that sex in the first place.
Abstinence, whether permanent or temporary, does not count as a 'purposeful act' because it is the
absence of the sexual act.
Obviously.
Why the dichotomy? It is important to maximize offspring WITH thought to the risks while engaging in a lifelong commitment, which is the goal of NFP.