This brings to mind an interesting question: is fertility always good?
If there are times when fertility is not good (and I’m proposing that there are), then it should be licit to suppress fertility, in the same way that one can morally suppress the immune system.
Depends on what you mean by fertility…meaning making babies or the potential for making babies? I’ll assume the former, correct me if I’m wrong.
Fertility in the making babies sense is always good, although there is the matter of degree. There are licit reasons for couples to decide when and how many children they have (although this can be morally very complicated). Limiting children to a certain number in order to maintain a certain lifestyle or other selfish reasons might be morally questionable (new BMW vs. baby).
On the other hand, if the mother has a medical condition that would put her life at risk during childbirth, that too is a factor; so is the financial stability of the household. Having way more children than you can support, perhaps for the selfish reasons of wanting more children to support you in your old age is morally questionable as well.
That’s not to say I’ve explained these examples well, but our fertility isn’t just there to make as many babies as possible. There are reasons to limit our fertility, in that sense, which brings us to “suppressing fertility”.
That’s where NFP comes in; while designed to “limit fertility”, NFP does not “suppress” fertility, as would the various means of contraception. The key point in Catholic Theology is that the couple not actively “suppress” their fertility.
Another dimention is that of marital chastity. In a sense, it limits fertility (100% effective

), but does not suppress fertility.
The real moral challenges come into play where a couple cannot effectively use NFP, as in cases where the woman has irregular cycles, or other medical considerations.