Wow, it’s quite wonderful to see that people think like you! I’ve always had this impression that NFP was all about
avoiding children. Abstinence is the best way to do that, and you would probably agree. But if you and God
want children, NFP would be very useful, as it was for you.
For the people I know, that is the most common. NFP is about
spacing children, first and foremost. But still the other question you ask is a valid question, basically my paraphrase would be if a couple is “done” having children shouldn’t the appropriate action be total abstinence? It is a good and valid question. My answer would most certainly be, no.
In addition to the very important marital bonding that LJN21 pointed out there are are even further reasons I would name. What periodic continence brings to marriage is
sacrifice. Total abstinence is a
total sacrifice. But we know, that after a while, total abstinence also becomes a habit. Periodic abstinence is very much like fasting. I know many post menopausal couples who practice periodic continence as a Lenten sacrifice. They don’t do it for the avoidance of pregnancy, (obviously.) They abstain as a sacrifice.
Secondly, if a couple has discerned they are “done,” then total abstinence guarantees that, since I don’t think our Lord is going to do another miraculous conception anytime soon. And who says they are done? It isn’t the “failure rate” (1%) of the methods of NFP to consider. It is the miracles rate of our Lord Almighty. Proper use of NFP is not just a child to child decision. It is a month to month discernment. And at it’s most holy use is a marital act to marital act discernment. In Humanae Vitae the wording is (paraphrased) ‘
exclusive use of the infertile phases is to be prayerfully considered.’ With total abstinence that discernment never happens again. The door is closed.
Now as I said, if another pregnancy meant
certain death for me and/or the baby, then total abstinence would be my only course of action. No one in their right mind would trust their entire existence to a flimsy piece of latex, a botched surgery, or an unreliable hormone alteration. That’s just plain crazy. But if it is only a
possible threat of death then I would still periodically abstain. Even with my health I have a statistically higher chance of getting hit by a bus than I do in dying in childbirth. Besides, I already got hit by a bus 20 years ago so the odds are with me on the childbirth thing.

(Don’t worry I was in a car and wearing my seatbelt.)
Basically, periodic continence is biblically, ‘by mutual consent’ and the time is spent in prayer. I like to remind my blessed friends who don’t have to use NFP, “When was the last time you abstained during infertility or fertility as a sacrifice?” I would sure feel very blessed if a non-NFP couple said to me, “LittleDeb, we abstained this month during
infertility as a sacrifice for those couples who can’t conceive. We devoted that time to prayer for the infertile.” I can’t tell you how much that would make my day!