Are there any cases in Cannon Law?
1. Both partners must agree. It cannot be unilateral. This is because the marriage right is "permanent, continuous and uninterrupted." That means that one marriage partner can never deny the marriage right to the other partner without committing a grave injustice unless there is some sufficient reason why it is inopportune to fulfill the marriage obligation at this time, but it must be fulfilled as soon as it becomes opportune.
I'm all for this. Another reason why communication is so important in a marriage: every couple has to come to a general agreement beforehand that should passion overtake them, they will graciously accept an outcome of pregnancy. This must be decided upon as a general rule of thumb, in advance.
2. Both partners must be able to fulfill the conditions without the temptation of falling into sin. If continence, even periodic continence, were a source of temptation towards masturbations, adultery, etc. for either partner, then NFP could not be attempted....
I'm all for this, too. But at the same time, it's more a matter of "grow the hell up." If a person is "at risk" to do any of these sinful things, what did they do before they got married? Is a man going to go to a prostitute because his wife is sick in bed for a week? Whether it's due to the flu, or NFP, a brief abstinence still = no sex. Such an "at risk" person is not mature enough to be married in the first place, IMO.
Not that I'm aware of, because it would be a matter for the "internal forum," i.e. an individual's conscience. Generally issues become matter for canon law when they become public. For example, an annulment is a public repudiation of what had been a publicly sworn commitment. But the decision to use NFP would rarely enter into the public arena or become a matter of canon law. However, the canon law that was in effect until 1983 did specifically state that the primary purpose of marriage is the procreation and education of children.