Again, Paul, it all boils down to intent. If you choose to abstain during fertility because you do not want to become pregnant, that would be sinful.
And I have a problem with that. What other voluntary act is sinful if not performed at a certain time?
In this instance, as in you can’t take the whole marriage as being open to life but every act has to be open to life, does a couple have to have marital relations
every day during the period they know is fertile? Is every other day sufficient? How about
once during the fertile period?
Once you say that the couple is obligated to have relations during the fertile period, you have to answer these questions. They seem silly, but that’s the logical end to the argument.
A serious reason to use NFP to avoid conception is not specified by the Church. There is no list. Yes, it can be abused, but, frankly, it
is subjective. If a woman feels that one more child would drive her over the edge, who are we to second guess her? If a couple doesn’t think they can afford another child, well, is the Church going to provide a financial planner?
As I say during these discussions about NFP abuse, come on, 3-5% of Catholics use NFP. Go pick on the other 95-97% that don’t.