If that were so, then infertile couples and couples beyond the childbearing years should be mandated to use something like an NFP cycle to regulate their sexual activity. It is possible for couples using NFP to be sexually selfish and lustful. NFP does not guarantee chastity in marriage. There are none too few cases in which a couple using NFP includes an unhealthy individual who prevails upon his or her spouse for intercourse during a fertile period. The consequence of this is bringing a child into an already-stressed or even abusive family system.
Sex is for married couples only.
Scripture and Tradition make this clear. Because sex promotes deep and profound bonding, it presumes the kind of trust and vulnerability that is free to emerge and grow only in a marriage commitment. Within the context of marriage, whether one is using NFP or other methods of avoiding conception, chastity cannot be determined by objective criteria. The virtue of chastity safeguards and promotes growth in married love, and so it can only be measured from the growth in generosity of the couple as individuals and as couple. The couple decides to have sex by mutual consent.