My daughter and future SIL just finished the natural family planning seminar. It was taught as part of their pre-marital counseling retreat. She said it was very helpful and offered more data points than the so-called rhythm method. When used correctly, which requires a great deal of self-control, I imagine, it’s about 86% effective. Not as good as artificial methods, but not bad for those trying to follow the church’s teachings. And I know a number of evangelicals who use the same approach. It’s not just Catholics.