SDG here (not Jimmy) with a (slanted) story from Scientific American that nevertheless offers some encouraging evidence regarding acceptance of the effectiveness of natural family planning among secular researchers.
Here’s their (slanted) headline:
Modified Rhythm Method Shown to Be as Effective as the Pill—But Who Has That Kind of Self-Control?
The slant is also evident through the story, from the "Vatican roulette" reference in the lede (opening paragraph) to one researcher’s disparaging remarks about the term "natural family planning":
"For many couples this is highly unnatural. ‘Natural’ is methods that you don’t have to think about, that allow you to be spontaneous…"
Sorry, but there’s nothing "natural" about latex barriers (which you certainly do "have to think about") or barrages of hormones specifically designed to short-circuit the natural functioning of a major bodily system (which you ought to be thinking about).
NFP, meanwhile, is entirely "natural" in the most meaningful and relevant sense: It accords with natural law, with the truth about human nature. It may or may not come “naturally” to couples raised in a sex-obsessed immediate-gratification contraceptive culture, but then neither do things like fidelity and lifelong commitment. Unsurprisingly, couples who do have "that kind of self-control" also turn out to be a lot better than their contracepting peers at the latter things too.