"For example, it is OK to prevent a pregnancy when probability of a birth defect is high, or there are grave health risks due to the age of the couple, or the couple truly cannot afford the child."
So then.......the difference is INTENT and NOT the method by which the intent is applied. If that's the case......a couple nearing the age of infertility is not in grave danger if they had a vasectomy, right?
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.