The fact that among the many things in my life, certain things I can choose and certain things I don’t have a choice is itself manifesting God’s will. If I am choosing between good or bad, then the fact that they both ultimately have very different consequences is itself manifesting God’s will. And very often, God also leaves it up to us to choose between two goods: e.g., marriage or religious life. In this case, He would reveal His desire for us, but would leave it up to us to choose. And if we choose even the lesser out of circumstantial reasons or even out of fear, if we still try to do good within that vocation, we know there would be a share of suffering that comes with failing God, but He would still bless us; and that, itself is also manifesting God’s will.
And in your example, if a person is called to marriage, it is God’s will and law that they are to be open to life. If this couple deliberately chooses not to be open to life, not because of sickness or finance or other serious factors, but simply out of selfishness or convenience, there is a consequence to that, and that is manifesting God’s will as well.
Nothing can happen without God’s will: either God posits a good, or He permits some evil to happen, usually because we choose them, so that He can bring something good out of it; and the example par excellence is the cross: we caused crucifixion, God took that up and brought us forgiveness and resurrection.