It may be worthwhile to note that the phrase “causis seriis” – serious causes – used in the paragraph on NFP is the same one used in the catechism in the paragraph on missing Sunday Mass.
2181 The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin.
Then look at the first time NFP was mentioned in a papal statement –
Pius X’s address to midwives (I find it goes more in detail than Humanae Vitae does, though the basic gist is the same):
Marriage obliges the partners to a state of life, which even as it confers certain rights so it also imposes the accomplishment of a positive work concerning the state itself. In such a case, the general principle may be applied that a positive action may be omitted if grave motives, independent of the good will of those who are obliged to perform it, show that its performance is inopportune, or prove that it may not be claimed with equal right by the petitioner—in this case, mankind.
The matrimonial contract, which confers on the married couple the right to satisfy the inclination of nature, constitutes them in a state of life, namely, the matrimonial state. Now, on married couples, who make use of the specific act of their state, nature and the Creator impose the function of providing for the preservation of mankind. This is the characteristic service which gives rise to the peculiar value of their state, the bonum prolis. The individual and society, the people and the State, the Church itself, depend for their existence, in the order established by God, on fruitful marriages. *Therefore, to embrace the matrimonial state, to use continually the faculty proper to such a state and lawful only therein, and, at the same time, to avoid its primary duty without a grave reason, would be a sin against the very nature of married life. *
Serious motives, such as those which not rarely arise from medical, eugenic, economic and social so-called “indications,” may exempt husband and wife from the obligatory, positive debt for a long period or even for the entire period of matrimonial life. From this it follows that the observance of the natural sterile periods may be lawful, from the moral viewpoint: and it is lawful in the conditions mentioned. If, however, according to a reasonable and equitable judgment, there are no such grave reasons either personal or deriving from exterior circumstances, the will to avoid the fecundity of their union, while continuing to satisfy to thee full their sensuality, can only be the result of a false appreciation of life and of motives foreign to sound ethical principles.
From this we can see that allowing God to send children – not just being theoretically open to life by not contracepting, but actually making conception possible – is a positive duty. The idea that “no third party can
make a couple engage in the marriage act” is thinking about it wrong. No one can make you go to church either. But there are duties we contract due to our state in life, and openness to children is a duty of marriage. Take St. Paul, who said, “Deprive not one another, except for a time, that you may dedicate yourselves to prayer.” St. Paul isn’t making you have sex – but he is telling you that it’s your duty.
Now, just as with Mass attendance, there are things that can release you from that duty. God does not ask us to fulfill every duty at every moment. Sometimes your child is sick and you can’t go to church. Sometimes it’s “oops, I overslept.” But you don’t have the intention to miss Mass without a good reason – if you do, that’s a sin.
Same holds true for using NFP. When you use it, you’re telling God, “We know we have this duty, to let you send children our way, but because of what’s going on in our life, we can’t fulfill that duty right now.” That’s totally valid – but you do have to have a good reason.
As far as what a good reason is – I could see health, finances, tough family situations (the latest child has health problems or a disability) as being good reasons. However, we can’t listen too closely to the spirit of the world in this. Baby magazines and nosy people in the grocery store tell you a dozen times a day that having more than a certain number is unreasonable, that you have to be able to afford a college fund for this child or you’re being irresponsible, that the world will get horribly overpopulated and that it’s ALL YOUR FAULT – that’s coming from an incorrect mentality, one spawned by the contraceptive culture. No one should ever make you feel guilty for letting God plan your family. He does know what He’s doing!
Okay, long long post. I’ve done some research on this before, so I wanted to share my sources and reasoning. The short form? There are lots of good reasons, but you do have to have one – not just an unselfish reason, but something serious enough to put a duty that you have as a married couple on hold.
All things considered, baby #3 was pretty easy on us. Hmmm…