As long as a married couple is using NFP and are okay with whatever potential kids Providence might send them, there's no obligation to try for children or to just go at it without NFP. There used to be a good number of couples who would marry with the determination not even to have sex, much less have children.
For example, the Blessed Martins, St. Therese's parents. They got married at a fairly advanced age for the day, and were married a couple of years in a celibate way before God let them know that He wanted them to consummate the marriage and have several kids.
If you're worried that you're not playing fair with God, or contrariwise, if you think God wants you two to try for kids, then sure, you two should pray about it.
But God isn't going to put a gun to your head and force you to have sex on fertile days. It's not a sin to not have sex, particularly if you two are in agreement as to the days you have a headache.
St. John Chrysostom says that the unitive aspect of sex is inherently procreative, because by coming together in one flesh, the two become one human being, one "new" image and likeness of God.
St. John Chrysostom on Theology of the Body, Part 1
. Part 2
Married couples without kids can also be "parents" by creating things or by helping people.