"There is *always* a possibility of conception."
Yes, we are on the same page! As a matter of fact, pregnancy is much more likely in artificial birth control then it is with NFP. As I've said, those that teach NFP assert the effectiveness of preventing pregnancy using said method.
I think that's a very unfortunate direction to be taking, and the director of the Couple-to-Couple League has recently addressed this question. The teaching of NFP has been driven by the fact that the majority of Catholic married couples do use artificial birth control, do drastically limit the number of their children. The majority of engaged couples plan to do the same. The thinking is that if NFP instruction takes the position of "same result, morally-approved (and healthier) means," eventually couples will come to see the blessing of an abundant family.
This has been my personal experience. My husband and I started using NFP for practical reasons (insurance didn't cover birth control pills), and over time came to see (numerous) children as the ultimate reward of our marriage.
As the article mentioned, and earlier comments emphasized, a big problem is that many, many couples come to marriage with absolutely no Catholic or Biblical foundation. Their beliefs, practices, and expectations are strictly a function of the culture. This formation should start at Baptism, but it isn't ... starting with NFP as "birth control" is at least starting *somewhere*!
You are quite correct on this point as well. Promoters of NFP never fail to trumpet the "effectiveness" of the method. The "possibility of conception" is said to be extremely remote, at least according to the literature of the Couple-to-Couple League and others of that sort. So that argument in favor of NFP is clearly specious.
Each married couple faces a fundamental choice to opt either for fruitfulness as God commanded them in the Garden of Eden, or to opt for "planned parenthood," whether it is accomplished via one means or another. Calling it "responsible parenthood" instead of "planned parenthood," and calling it "natural" family planning instead of "artificial" birth control does not alter the reality. I must choose whether I will accept children from God with joy, or whether I believe that I can do better than Him by "planning" it myself.