I am a Billings Method Teacher of 1980's. I do not teach the way the curriculum is set up: Secular with no God, or of the attitude that a young couple can put off having children till their college is finished or such. (couple to couple league brags in their newsletters) I can say that it was a disappointment to see the beauty of God in the anatomy and physiology to fit the ways of "man's idea".
Sin happens when the Knowledge given to man by God is man's way and not God's. That is the attitude/heart which will be judged.
In 1980, I had for the first time a video of Billings and the first book. I was age 27, young enough not see all. I had classes with the Billings, twice. I saw nothing wrong with the curriculum, but it was there, no God was mentioned and it was done very cleverly. Who did that, I do not know. It took me time, years to see that. In 1990, I was offered to please file for grant money and then I knew there was trouble, because I knew what grants were. I was offered to read grants by the governor of AZ. He wanted to know who was bringing sex ed. into the state when it was against the laws.
While that was brewing, I ran into a group: Sympto-thermal. Well, they did not like Billings, in fact the head person teaching under the dioceses told me there was a blood bath with them. I also got a bad "attitude" with this head person of ST. "We teach better than you!" Better how? Oh, we do all of these checks of the anatomy. Well, to make a long story short I happened to run into one of these users and this woman said to me, "How can you go mucus only?" I said, because that is what God gave us and that is all that is needed. Thermometer is of man and when you lean too much on so many signs, it can cause aggravation. Besides, you were taught that thermometer give signs in pre menopause.
So, with those issues circulating in my head and soul, what resulted. First of all the Billings were very hurt and disappointed when they found the knowledge of Billings go with the taking of federal grants $$$. Dr. Billings said in front of many people at one of my conferences, "This (Billings knowledge to be taught) is God's!"
I knew what he was getting across. He was very unhappy.
Then I found Planned Parenthood teaching Billings by all messed up. They taught Hi-tech Rhythm, their words. They said, " Most couples do not wait so use condoms and such. See! And what would be PP's results. I know, pregnancy and PP had high hopes that their way of teaching would result with abortions and also what God gave would be back to what it was in the minds of man, just another rhythm to disregard.
And who brought sex ed. into the state of AZ. The dioceses. Wow! Did I get a wake up call! Boy, did I need to pray about the whole issue. And I still do. Then I came across how Communism works when reading grants known as KAB. KAB is used to change the thinking of men. KAB =Knowledge, attitude, behavior. Then I read that in our catholicism we do the same and it equals up to sin or to God Glory. With knowledge we know love and serve God in this world for us to meet in Heaven. So, behavior would be for me to save my soul.
For communism, to change knowledge OR take truth and twist it, to change attitude, to change behavior. This is Artificial man made birth control for just a starter. You can take KAB and see how it fits communistic ways.
So, after much prayer and I still pray about this, IMO, what God has made is most beautiful. Depending how it goes to attitude and behaviors is where sin is. We know God will judge the heart/soul.
IMO there will be serious reasons. But it is heavily on attitude that needs to be checked!