Here it is not a question of chastity in the narrow sense of a virtue as a disposition to good behavior. Rather, chastity is considered in the broad sense as "the successful integration of sexuality within the person" (Catechism, No. 2337). . .
The theology of the body is centered in the gift of self. Only the chaste person can make such a gift. For spouses, this gift is embodied and expressed in the act of conjugal union. But no one can give what he does not first possess. It is precisely the mastery of oneself, the self-possession which comes through chastity, that enables men and women to be a gift to one another.
I'm going to remember this the next time I want to explain why it's so much better to preserve sexual expressions of love for marriage.