I would say being “open to life” builds strong marriages. Whether that openness includes NFP or not is irrelevant.
NFP is not without it’s challenges. Truly it is a great gift for families to be able to space and limit children when necessary. But it certainly requires sacrifice and hardships which (along with any trials) can make a marriage stronger. It is, however, worthwhile to note that NFP is not always necessary and a great many people have wonderful marriages simply leaving such things in the hands of God. NFP should not be a way of life. It is a tool to be used when circumstances determine that it is necessary not something that should be used widely everywhere and at all times.