A couple that is practicing NFP may be open to the possibility of conception but their reason for practicing it is to prevent conception. This goes against the Catholic church's teachings that "every sperm is sacred".
And it is very possible that women can still go through their un-medicated menstrual cycle even if an egg is fertilized.
Oh good; I was worried that you were seeking information and I was doing a poor job of supplying it. While I am a new Catholic I must say that I've never come across the teachings you've referenced. Perhaps you are misinformed? I've already stated that NFP can be practiced sinfully.
And it is very possible that women can still go through their un-medicated menstrual cycle even if an egg is fertilized.
Yes, miscarriages are sadly very common.
You're getting your information on Catholic doctrine from a satirical song in a Monty Python movie?
Monty Python films are not official statements of Catholic doctrine. (One would think that this would never need to be stated.)
A couple using NFP deposits the "sacred sperm" where it is supposed to go. And no chemical or physical barriers have been placed to block the action of either the man's or the woman's body.
So, you fail Catholic theology. Try a book sometime.
SD