The difference is both intent and the method. The use of NFP to avoid pregancy is invalid if the intent is invalid, namely if the intent is, for example, to have two uninterrupted careers.
The use of contraception is invalid method even when the intent, as with an older couple, is valid.
Vasectomy, on top of everything else, is self-mutilation, which is an additional sin.
This moral teaching about intent and method should not surprise anyone, as we are familiar with it in other areas of life. For example, we don't steal money (improper method of getting rich) even though the intent might be proper, to pay for the college. Nor do we display wealth in order to impress the neighbor (improper intent) even though the method of making that money, engaging in honest commerce, is proper.
The difference is roughly, between coming to church to pray and coming to church, turning your back to the altar, plugging in earphones and listening to Smashing Pumpkins. The intent in both cases is to be in church...
"The use of NFP to avoid pregancy is invalid if the intent is invalid, namely if the intent is, for example, to have two uninterrupted careers."
What if the intent is to space the children? Is that valid? What if there are only two bedrooms in the house and the couple is going on the fourth child? Is that valid? What if the husband is expecting a decrease in pay? Is that valid? What if the wife is having trouble raising so many children, she finds the task daunting? Is that valid? What if the couple is in the middle of a move to another city soon? Is that valid? I'm not trying to be cute, I'm really not. I just have a problem, to a very large degree, with Letter of the law Catholics, and the NFP issue brings it to the fore more than any other Catholic issue I can think of. It is the splitting of hairs and that's where our discussion has gone here. No one, as much as they have tried, has put my questions to bed in a very practical and reasonable content which would squash all my doubts. I have studied this NFP issue intensively and it's the practical application that just gives me fits. I know I'm supposed to buy into NFP as a good Catholic and to fortify my faith with it's practice. But on an intellectual basis I am not getting it. I understand "Do NOT KILL." , "Do Not steal" and the other 99.99% of our simple and strighforward Roman Catholic Faith." I understand it's moral applications, the intent, the consequences, etc. It all makes sense. However, this one issue still confounds me to no end. Maybe one day the proverbial light bulb will illuminate in my soul.