This drive is meant to be lived out and enjoyed by man and woman in the permanent loving union called marriage. We should be able to know this by using our reason. To make it clearer, God decided to make it part of his Revelation. Unnatural When we make our sexual drive into an end in itself or go against its procreative purpose, we are acting unnaturally, for not doing what God intended. Violating his plan can never really fulfill us or make us truly happy.
Contraception is an example of an unnatural practice.
Contraception is immoral in that it shows a lack of trust in God's loving care. In contracepting, a couple relies not on God and their own grace-assisted ability to abstain for a while, but put their trust in contraceptive means. Couples who sever the two ends of marriage block the natural process of conception. They do all in thier power to make intercourse incapable of conception. NFP would be immoral if practiced for purely selfish motives.