The physical act of intercourse, reserved only for marriage, is proper when it is an expression of love that is open to the possibility of new human life. An act of intercourse between a married man and woman is a true expression of love when there is a total giving of self. Total giving of self includes giving and receiving the gift of fertility. That is what is meant by living marital chastity. Any act of suppressing, blocking or destroying the gift of fertility or destroying new human life once begun is an act against love and life.
Although couples are called to be generous to new life, there are times within married life when spacing or limiting the number of children is prudent and responsible. A married couple discerns this responsibility in a prayerful and selfless way. Serious reasons for spacing or limiting children could be physical (health), psychological (mental health), economic (financial) and social. Serious reasons are determined within the context of valuing the couple's duties toward God, themselves, their family and society in a correct ordering of values. For example, a couple may want to avoid pregnancy when the husband is out of work, if another child has a serious illness or if a parent is unable to cope with the children already born.
When married couples have discerned that there is a need to space or limit children, the means to do so must be truthful to love, the integrity of the sexual act and to the gift of fertility. Medical science has taught us that there are times within a woman's monthly cycle that are fertile and times that are infertile. Natural family planning is a means by which a couple learns how to monitor the woman's monthly cycle and to interpret the natural signs that tell them when she is fertile or not.
If the couple has serious reasons to avoid pregnancy they then periodically abstain from intercourse and genital contact on the fertile times of the cycle. During this time, couples are challenged to express their intimacy in nongenital ways, such as through spiritual (praying together), physical (taking walks together, just holding one another), intellectual (sharing a good book), communicative (talking and listening to each others' needs, desires and fears) and emotional (laughing, crying) needs. Although this may be difficult and at times seem impossible, God does not ask the impossible and will be with the couple in difficult times in a special way.
The practice of NFP and periodic abstinence, far from harming married love, actually confers upon married life a higher human value. The many positive benefits of being chaste within marriage and practicing NFP include a better understanding of your fertility, increased communication, self-mastery of one's sexual desires, a greater generosity toward new human life and an openness to God's will.