So, I was thinking about an aspect of NFP that doesn't seem quite right if NFP is really acceptable. And, that is that it requires the agreement of both spouses for it not to be sinful. And, I find that interesting. I will give a number of thoughts or reasons why. First, it does not require the consent of both husband and wife for marital relations. If there is no impediment like grave sins against the 6th commandment, then each spouse has a right to the marital debt. So, this agreement of the spouses turned on its head, is not consistent. Because, it doesn't require consent of both spouses for the marital act to be lawful, in fact if one does not agree, it can easily be unlawful and sinful on the part of the denying spouse. That is interesting.
Turned back onto NFP, if it is sinful, which I think it is, then just because both spouses agree to the sin, does not make it not sinful anymore. And, if that sounds familiar, that is because it is the MO of relativism in the church(aka modernism). What is truth? The modernist would say that truth is whatever I want to believe, or whatever the majority of people believe in, or whatever the person who has the most money believes in. That is where you find truth the relativist will say. In the past it used the vehicle of probabalism, which morphed into collegiality at v2, and in its current most liberal form, it is a collegiality across all levels of a catholic's state in life. Hierarchy is no longer, and morality is to be subject to the mind of the married woman. That is modernism. And, we know what happened in the garden of eden, so that is a flop.
Sin doesn't not become sin just because more or "enough" people believe it is not a sin. Morality is objective. Morality, in this case that of NFP, is not subject(ive) to the consent of both spouses. It cannot be if it is a moral issue, and not just a prudential decision. If it is truly a prudential decision, then it could be subject to one of the spouses. Because, the man is the head of the house, and the woman is the heart. The man makes the prudential decisions. And, the woman breathes life into them. But, the church has said that NFP is sinful if only dependent on one spouse. So, it is not a prudential decision.
Scripture says the married man is divided between pleasing God and pleasing his wife. That means that man an woman cannot see eye to eye if a moral issue is dependent entirely on them. Because, the married man is divided. And, we know that the woman is divided. She is made from mans side, not knowing God as Adam does. And, if NFP is not a prudential decision, which it isn't, as demonstrated, then it is a moral decision. And, that means that Christ decides on it. In sum, the marital act is surrounded on both sides by moral decisions, not prudential decisions. It is surrounded by Christ and morality, consistent with any sacrament. Consent of the spouses gives it away.