We have to be very careful with specifics when talking about NFP for other people.
Leaving it at serious reason is fine, but needs to be understood in light of 2 things - because people get confused thinking someone else is being authoritative where the responsibly lies with each couple -
~Not having sex is not a sin
~The responsibility and authority of evaluating the serious matter lies with the married couple. ( the church uses general buckets for guidance)
I am not sure I understand your statement - not having sex is not a sin. If one uses NFP for a non-serious reason and because NFP shows it is a “risky” day a couple decides based on NFP not to have sex then it very well could be sinful. If it is not sinful then logic would say reserving NFP for serious reasons would be useless.
Your statement concerning the couple being the point of decision is not in keeping with Church teaching. I would say that evaluating a situation starts (HV is below showing explicitly it is not up to the couple alone) with the couple IF they have an informed conscience, which means they fully understand church teaching and use those teachings to guide their conscience. A Conscience that is informed can’t reach a decision that is against Church teaching. Of course the Catechism also covers what happens when someone doesn’t look into what the church teaches when they should. This is a perfect example of a situation where that may come up often. Since a couple is well aware that the NFP document say serious reasons if they choose to guess at serious reason rather than research and see what serious reasons are ( Serious reasons mean important, or non-trivial, reasons, deriving “from the physical or psychological conditions of husband and wife, or from external conditions” (HV 16). If therefore there are well-grounded reasons for spacing births, arising from the physical or psychological condition of husband or wife, or from external circumstances, the Church teaches that married people may then take advantage of the natural cycles immanent in the reproductive system and engage in marital intercourse only during those times that are infertile, thus controlling birth in a way which does not in the least offend the moral principles which We have just explained. (HV20) and in more detail here - which specifically says it is not up to the couple…
Responsible parenthood, as we use the term here, has one further essential aspect of paramount importance. It concerns the objective moral order which was established by God, and of which a right conscience is the true interpreter. In a word, the exercise of responsible parenthood requires that husband and wife, keeping a right order of priorities, recognize their own duties toward God, themselves, their families and human society.
From this it follows that they are not free to act as they choose in the service of transmitting life, as if it were wholly up to them to decide what is the right course to follow. On the contrary, they are bound to ensure that what they do corresponds to the will of God the Creator. The very nature of marriage and its use makes His will clear, while the constant teaching of the Church spells it out. (HV10) _
w2.vatican.va/content/paul-vi/en/encyclicals/documents/hf_p-vi_enc_25071968_humanae-vitae.html sorry NFP’ers now you can’t say you do not know…
To make no mistake, misusing NFP is not as serious sin as the Pill or other methods. It would fall into the category of selfishness or disobedience… A decision to abstain from sex because you may have a kid when no serious reason is present is not being fully open to each other. It literally is saying… I don’t want to have sex because it may cause a baby. I also doubt saving up for a vacation, so you can upgrade your car to a BMW or afford a country club membership would be seen as serious reasons. I have had detailed discussions with a friend who has been his parish’s leader of NFP with his wife for 15 years who wasn’t clear on this. When he looked into it it was eye opening. Many are taught that NFP is always OK. That is incorrect and all approved literature for training includes the above language somewhere. (if it didn’t it would not be aligned with church teaching) Of course I was part of the marriage encounter generation of the 1980’s where well meaning priest actually told our group that the pill was fine, as long as we “wanted kids and had them later.” Hope this helps those making moral decisions.