My OP is quite a rough draft, and this subject is a rough draft, so don't think I am jumping to any conclusion despite sounding biased. I am just trying to argue to a bias, because there is no need to argue its opposite. Its opposite seems to have nothing but support.
Songbird - when I watched recent novus ordo promotionals about NFP they mentioned mucus monitoring as a sign or symptom and it seemed the primary sign they monitored. And, these vegan ladies mentioned that they have reduced signs or symptoms. And, like materdominici mentions I don't think nfp is like ticking days off of a calendar. So, combine reduced signs(mucus) with irregularity, how would one feel confident with this system? One women who went onto the vegan diet mentioned that here pre vegan cycle negatives(her period) were very intense, and that was perhaps a reason why she sought on a vegan diet. Almost all of them report not having any negatives or pain on a vegan diet.
Pax Vobis - one female mentioned how all of these artificial hormones are a big causer. I ate an animal free diet for many years. And, I was able to do it because it is quite simple when clear cut. But, to go organic meat and dairy and all organic for that matter I am not able to do. But, I would love to experience its effects. Because, I am biased against animal products when I am only really familiar with common meat and dairy, and as you say, they are pumped with chemicals and what not. I almost feel it disqualifies me from judging. I do know that I become quite defenseless to cravings when I eat any modern processed foods or modern meat/dairy. I wonder how my cravings would be if I ate organic meat and dairy. I think I could test that. I do know that on a non organic vegan diet, I was free from cravings. Hence, that is why I was able to do it for years.
Materdominici -I know it is a stretch. But, if it can strengthen an argument, then I will be glad to explore it.
My thinking is that there is a reason NFP(which touts like a 99% success rate) only came about recently. Previous generations were very clever. And, nfp seems quite simple. You would think that this knowledge would have come about earlier. And, this knowledge has come about by observation. Vegans say a plant based natural fat diet reduces or eliminates signs and symptoms(observation?). A natural plant based with more natural fats would be closer or in fact the diet of past ages. Perhaps that means you can associate the two. Woman of the past did not have signs an symptoms enabling or suggesting the knowledge of NFP.
Dr. John Mcdougall in his seminars would always talk about how high fat diets and/or fruiticians(people who only eat fruit) is not a traditional diet because it couldn't exist in past ages. And, he argues against them. He argues that the traditional diet is the starchavore diet(eating plant based starches). Catholics are traditionalists. If diet is proximate to a females signs and symptoms, and our diet is proven not traditional, why should we change our morality to fit new females signs and symptoms? Our morality is supposed to be traditional, preserved in the deposit of the faith.
Perhaps we ought to find out if the amish are practicing NFP. Because, they consume a lot of organic fats(milk) I think. It would be neat to find out about their females cycles and signs and symptoms. But, they don't even allow people to take photographs of them. So, I would think their women would remain "hidden" in this manner. But, that is good, so I will prefer that.