[wideawake]

So a RCC married couple could live their entire married lives using the natural method of birth control with great success and NEVER have a child.

No.

Couples who are capable of having children have a moral obligation to have children.

NFP is intended to provide a respite if necessary in a larger context of having children.

For example, my friend who was advised by her doctor after a couple of miscarriages to take an 18-24 month break between pregnancies in order to reduce the chance of further miscarriages.

That is an appropriate context.

[Running On Empty]

Correct. There are frequently cases where, for a serious medical reason, a woman who already has children to care for, may need to avoid another pregnancy for a limited period of time.

That is one value of knowing NFP. On the the other hand, NFP can be helpful when couples want to achieve a pregnancy.