Pope John Paul II said that "Chastity can only be thought of in association with the virtue of love." So simple, yet it sets chastity in the right light. Without love, chastity looks a lot like prudishness or repression. If we do not understand what it means to love then we will not understand chastity. Love seeks what is best for the beloved and is willing to make sacrifices for them. Sex outside marriage puts the beloved at risk of: out-of-wedlock pregnancy, STDs, psychological and emotional pain, spiritual void, bad reputation, loss of trust from parents and others, stress and anxiety, loss of dreams and goals, etc. Love would never place the beloved at risk.
Jason Evert says that chastity is a virtue, just like "courage" or "honesty," that relates to our sexuality. It means we have the strength to use our gift of sex in accord with God's plan.