NFP does not allow for sexual "spontaneity."
Most of the time, "spontaneity" in sex is itself a myth!
Modern culture is awash in sexual messages. This may fool us into thinking that "everyone" is having sex as often as possible and always "spontaneously!" Even married couples may fall into this trap. Or, they may think that their sex lives would be more spontaneous "if only" their spouse wanted sex at the same time they did. The reality is that most marital sexual encounters are planned, or at least happen in situations favorable to love-making that are setup by agreement regardless of the family planning method used. Otherwise, in the press of daily life with jobs, household chores, social commitments, children's demands, etc., a husband and wife would rarely have sex!
It's not necessarily bad news that married couples often plan on a time to have sexual relations. A loving invitation given in advance means a time of healthy anticipation for both husband and wife. For NFP couples who are trying to postpone a pregnancy, such an invitation in the days of sexual abstinence can mean living a "chaste courtship" that will be followed by a "mini-honeymoon." NFP couples often talk about how the times of sexual abstinence have helped them deepen their expressions of love for each other through loving gestures, "date nights," significant conversations and so forth. This is not to say that the times of abstinence are not challenging. They can be! With a positive attitude and living through it together, husband and wife can use periodic sexual abstinence to grow individually and as a couple. Such self-mastery fosters authentic freedom where one's desires are put in service to the other—a necessary ingredient for marriage! Real sexual spontaneity depends upon real freedom—and NFP fosters such freedom.