Miller also pointed to the language used by John Paul II regarding family planning. He said the late pope's word choice leaned towards "unselfish reasons."
"In other words, you get the idea that if a reason is an unselfish one, if it doesn't have to do with hedonistic wishes or something like that, then it's probably a serious reason."
In his encyclical, Pope Paul VI gave four broad categories that qualify as serious reasons to avoid potentially procreative acts – medical, psychological, economic, and social reasons.
Miller said if the medical or psychological risks of pregnancy are more than trivial, they qualify as serious reasons. This could mean a physical or mental strain on the mother, he said, but also health issues for other members of the family.
Using NFP to attempt avoiding or delaying pregnancy is acceptable "if having another child, at least for a time, is going to impose significant health risks on typically the mother, but possibly on some other family member," he said.
"Maybe there is already a child in the family who has health problems who would be harder to afford to care for that child if you have another child," he added.
Miller also pointed to the economic factors that could qualify as serious reasons to delay pregnancy through natural family planning, among them the likelihood that additional children would restrict a family's basic necessities, including food, shelter, clothing, and education.
"One way I like to look at it would be to say if having another child would put the family in the sort of economic situation that if it were the result of, let's say, a low wage would be called an unjust situation by the Church. I would say that counts as a serious economic reason for using natural family planning."
He said the last category, social reasons, is not as clearly defined by the Church, but he said it might include those situations in which a family could better serve the common good by having a "somewhat of a smaller family."
"Some families, in response to the teachings of the Church in places like Familaris consortio, wish to serve society in some special way, maybe like reach out to the needy in society," said Miller.
These reasons, he said, certainly do not require couples to refrain from procreation, he said, noting there is no flowchart or algorithm for decision-making.
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Rather, he said the decision should be made with honesty and prayer, using moral reasoning over moral rationalizing.
"First of all a couple has to make sure they are being honest with themselves, make sure they are reasoning honestly about their situation and not rationalizing," he said. "Secondly, I would also say…do your moral reasoning in light of your relationship with God. It should be a prayerful as well as an intellectually honest discernment."
Miller cautioned that couples ought "not to be scrupulous in these matters" if prayer and honesty accompany their decision. He said that unlike artificial contraception, NFP calls couples naturally to be sacrificial.
"I would also even say that, if a couple used natural family planning, the self-mastery and the discipline, that the Church has said comes with use of natural family planning, the more it is likely that as time goes by they will develop even more the virtues of generosity and selflessness."