If by sexual spontaneity one means loving gestures, caresses, kisses, loving words, etc. In fact, NFP encourages couples to explore ways to express their love to one another because sexual intercourse is not always possible. And the spontaneity with which a couples reunites in lovemaking each cycle is something NFP couples finds to be very special. Remember, chemical contraceptives carry heavy baggage in terms of contraindications and side effects. The Catholic Church wants people to have as many babies as possible.
Responsible parenthood is lived out within the structures that god has established in human nature.
That is why the Church teaches that couples must not actively intervene to separate their fertility from their bodily union. To do so is to show disrespect for an important gift of the Creator. Sex is a private decisions between to people. Sexual intercourse is not only eminently personal, between a man and a woman, but also has a public and even universal dimension. Human reproduction orients sexual activity to the future of the world.