What the Church teaches is not purity (in the way Evangelicals use the word), but chastity. Chastity isn’t something that you start out with and then can lose forever with one mistake. It is a virtue that takes prayer and effort to grow into. All sins can be forgiven through the sacraments. People like St. Augustine, who so famously said “Lord give me chastity, but not yet” can become a saint. The vocation of marriage, like the call to holiness, involves prayer, frequent sacraments, and hard work. Before marriage, we are still called to holiness, meaning we embrace virtue and avoid sin. We embrace the virtue of chastity, which includes abstinence for the unmarried, and avoid sin, here being lust. Within marriage, guess what? We are still called to the virtue of chastity. We aren’t called to permanent abstinence anymore, but we still need to be virtuous. In marriage, both partners need to lead each other to the Lord.
— SMP501, via Reddit