I would say that every couple who is, in fact, using NFP in the way you’re describing is trusting God. It’s the “if” in the bolded part above where the issue gets tripped up, I think. It’s one of those things, I’ve said it before I know but the couples have to be completely honest with themselves since the Church has essentially left it up to their own discernment. What I think some of us are talking about, (using an online example here) is the number and frequency of threads on this forum, for instance, where people will post about wanting to use NFP to avoid because they want to start their marriage off with several years of no children just so they can have fun and “get to know eachother” more. And things like nicer cars and better vacations are mentioned, etc. And then these same posters will justify themselves by saying that this is what they have “been called” to do, and who are we to say that their issues aren’t “grave and serious”?? etc. And indeed, we can’t. Not really. And even more than the online examples are the IRL encounters many of us have had with family, friends, church acquaintances, etc. who talk about their own experiences and feel free to comment on ours. I had no opinion about any of this before I got engaged and started seeing it/hearing about it for myself.
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I just wanted to touch on this part.
I think NFP(to avoid) users mostly do so because they want to do the right thing in the eyes of the Lord, learned through the Church. I assume that NFP’ers are being as obedient to the Church as possible. I assume that because, our society is bombarded with the message of contraceptives. There is a commercial that comes on for an IUD. It shows a frazzled mom with two naughty kids, and a Dad reading the newspaper as his kids play. She says, 2 is enough.
I’ve been married almost 13 years. We haven’t used contraceptives for nine of those years.
We have regretfully used contraceptives…partly in ignorance, mostly for convenience. I personally see the difference between using contraceptives and using NFP.
When using contraceptives, the “marital debt” (love that phrase, btw) is always expected to be delivered. But payment is in the form of a "check with non sufficient funds " , the removal of our fertility. The marital embrace had no consequences. If you ever watched Mass on EWTN, Father Shaughnessy (no longer there) gave a homily on contraception. He called it, mutual masturbation.

Unfortunately, I have the personal knowledge that, yep, that’s what it is.
In the beginning of our marriage, we fought about sex often…basically the frequency. There is a weird disconnect about wanting more frequency while simultaneously not wanting a child.
If most NFP’ers were being selfish in their reasons to abstain…that selfishness would rear it’s ugly head pretty quickly. When a couple loves one another, truthfully and honestly , it is a BIG SACRIFICE, to abstain. It’s not like there is no desire for each other. The desire is still there…and probably more so on the days we should avoid. But we sacrifice that longing, and we offer it up, with the hope that this is what God wants for us.
We’ve been selfish…I honestly can say, we are not being selfish now. When we were contraception it was, “I don’t want to be pregnant. I don’t want a baby.”
Now when we avoid it’s because we are thinking of all the possibilities, and how we need to be prudent, for our own health, for the health of our future baby, and for our family life in general. But, if it were to happen that we should conceive agian, that baby would be soooo loved and welcomed.
I also like to think this. My DH and I are faaaaaaar from being the perfect Catholic couple.
I’m sure there are many more devout couples…and more devout couples using NFP than us. If we aren’t being selfish, than I can assume that they are not either.
Hope that makes sense, sorry for the book.