God wants his prerogative in deciding to bring a child into this world. NFP leaves God his prerogative while birth control does not. Here is an analogy given by Dr. Douglas Bushman, STL that many find helpful:
Imagine your sexuality as an ice fishing house. If you live up north, when lakes freeze over people will build little shacks called ice fishing houses inside of which they will cut a hole in the ice and sit on benches around the hole and fish. This shelters them from the elements outside:
So God creates this small “arena” of sexuality where couples go to renew their marriage vows. One of his rules is: You can come and go as you please, but every month for a few days I will open the door and come into the fishing house and be present. If you choose to come in and “fish” while I’m there, I may choose to make a baby. That’s up to me to make that choice, but I want that prerogative. If I’m not there, there’s no chance a baby can be made. If you’re not there, there’s also no baby. But you can come and go as you please because that’s the way I created your fertility. I just want my right to be here respected.
Given this analogy, a couple using NFP goes into the fishing house only when they think God will be absent. God still comes and goes when he wants. The NFP couple is still using the ice fishing house according to the way God has set it up and they are not interfering with God’s entry or exit to the ice fishing house. However, since they are not all three present at the same time, no baby comes.
Now, given this analogy, a couple using artificial birth control would go into the ice fishing house and lock the door as they enter and then after they leave. God is denied his right to enter into that realm of fertility. They are actively excluding God from the arena when they are in there and they are assuring that God will not be there when they re-enter their house. They co-opt the fishing house as their own and ignore the God outside knocking at the door. no baby comes because not all three are present. True, sometimes birth control may fail and a surprise pregnancy occurs, but this would be analogous to God breaking down the door while the couple is “fishing” inside. It doesn’t change the fact that the couple is still trying to bar God’s entry into the house he created.
A couple where the wife has finished menopause, in this analogy, would be where God hangs up a sign saying, “I’m finished visiting”, and he leaves the door unlocked. Couples can still come in and “fish”, but God will not come and visit, by his design, and create any more children while the couple is there.
The guiding principle here is: Are you denying God entry into your (His) fishing house? That is what being open to procreation is about. It is not that you are intending to have a baby each time you have sex. It is whether or not you are being open to God’s entry into this realm of fertility.