Both couples perverted their marriages differently. Couple A is deliberately childless. This is against the Church's teaching even if they used NFP. The Church only allows NFP for the purposes of pregancy prevention if prudent consideration exists to avoid a pregnancy at that particular time. For example, it is OK to prevent a pregnancy when probability of a birth defect is high, or there are grave health risks due to the age of the couple, or the couple truly cannot afford the child. It is also OK to delay a pregnancy for health or economic reason. It is not OK to prevent a pregnancy for frivolous reasons such as lack of affection for children or desire to maintain two careers, regardless of the method.
Couple B resisted the purpose of their marriage by denying themselves the gifts of unitive and procreative intercourse. Despite God's steering them in the right direction with the unintended pregnancies, they refuse to hear His command, They insult their children by referring to them as mistakes (whether or not they use the word themselves, their life objectively treats children as mistakes of contraception). The happy outcome of four children does not accrue to their merit because they resisted God's procreative work in them.
"For example, it is OK to prevent a pregnancy when probability of a birth defect is high, or there are grave health risks due to the age of the couple, or the couple truly cannot afford the child."
So then.......the difference is INTENT and NOT the method by which the intent is applied. If that's the case......a couple nearing the age of infertility is not in grave danger if they had a vasectomy, right?