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Couple "A" practices NFP from the beginning of their marriage to the end of their fertile lives never having conceived, never having bore a child but having followed the letter of the law of the Catholic Faith to a "T." Couple "B" practices interrupted coitus during every single love making act. During the course of their lives they have 4 mistakes and bring each of these pregnancies to full term and birth. They raise the children happily ever after and those children go on to give them grandchildren. Which couple has done God's work?

Neither.

Couple A is especially abominable because they obviously had an intention to avoid all conception of children, which effectively renders their marriage null and void, since they effectively make themselves eunuchs.

There is an obligation for married couples to have children. This is not a part of sexual morality, but social justice arising from obligations towards society - "Be frutiful and multiply". Catholic Moralists had much discussion of this question in the middle of the last century. In North America, they came to the conclusion that there is a requirement to at least have 4-5 children if possible given the age of the couple at marriage. In Europe, they came to the conclusion that a couple should not abstain from allowing conception for a period of longer than 4-5 years. Both sets of moralists held this out as proper conditions only for our time and lands, noting they might be different in other times and other places. For example, in the time of the Holy Fathers of the Church while Rome still ruled the world, it was quite common for a couple to only have 1-3 children and then to abstain from sexual intercourse for the rest of their marriage to allow greater devotion to prayer.