The Catholic Church is often accused of being legalistic or hypocritical in its condemnation of contraception but allowance of natural family planning to space pregnancies when there is a grave reason to do so. People say that it's the same end, so why do the means matter? You wanna know the difference?
The invitation for God to change your plans if he so desires. He could, for instance, let you ovulate 5 days sooner than you usually do. Bam! Blessed with a child!
If I was using contraception I would virtually exclude the possibility from the the table altogether, I would be equally sterile all month long. I would be engaging in the sexual act which was meant for both the union of the spouses and the procreation of children in a way that excludes one or both God-given functions...
...And I would be bereft of this little bean who is currently taking up a comfy residence in my uterus, and then how much the poorer would I be?