It is intended to prevent the coming into being of another person. Its action is to separate, or suppress. That is radically different from being a good steward over one's fertility and co-operating with God and His design to space and limit births in a marriage. The latter perspective honors God as author of life, the former does not. This is a very reasonable question.
Physical acts express and affect our very being.
NFP allows us to live our beliefs. It encourages a sexual spirituality in married life. By practicing periodic abstinence when trying to avoid or postpone pregnancy, couples can take a careful look at how they love each other. They can become conscious of trying to approach every aspect of life without covetousness or domination. There is no room for greed and exploitation when one lives chastely, because chastity consists in the long-term integration of one's thoughts, feelings, and actions in a way that values, esteems, and respects the dignity of the created order, others, and oneself.