I have a friend who is struggling with scrupulosity, and she contacted Most Holy Family Monastery, who said that taking part in natural family planning is a sin, along with all forms of contraception. Now, I don't endorse contraception (birth control pills, condoms) but what is wrong with having sex when your bodies won't conceive?
Say I'm married and my wife and I have a few kids already, but we still decide to have sex without contraception, but plan it during her "time of the month..." This is natural, and I think Pius XI condemned things that go against nature, so how could this be a sin?