One additional point that I want to add:
I don’t think that people should second-guess a couple’s decision to use NFP, except perhaps in extreme cases – such as if a couple says “we plan to use NFP to avoid ever having children, because we want to live a rich and luxurious lifestyle, and kids will get in the way of that.” Of course, I have never heard of a couple using NFP for such an extreme reason, so in practice, I don’t think that people should second-guess. The decision to use NFP is a personal one, that should be between the couple and God, and not subject to the approval of others.
However, I can also sympathize with the tendency to want to second-guess. Now that my wife and I truly have to avoid another pregnancy for the foreseeable future, I feel sadness and loss at the thought that we may never have another child. Even though we are blessed with four children already, I grieve for the possible premature end of our child-bearing years. And I am sure that what I feel absolutely pales in comparison to what infertile couples experience.
So if someone is in that state of mind where they long for more children but can’t have them (whether due to infertility, or health problems, or past choices such as sterilization, or an unwilling spouse), that person may have a strong emotional reaction if they perceive that a couple may be avoiding children for selfish reasons. Such a person might easily think, “Hey, here I am praying every day, practically begging God to bless us with another child, and you choose not to have one just because [insert perceived selfish reason].” That emotional reaction is probably not reasonable or charitable, and it is almost certainly not correct to act on it by saying something (because we may be completely wrong about their reasons being selfish, and because it’s none of our business). But nevetheless I do understand where such a reaction could come from.