I have two sons and four daughters. I have come to the conclusion that the only way to teach chastity that will form a foundation that will last is to highlight the principle that the central goal of mortality is to become complete and whole (perfectly united with another person - physically and emotionally), and that such a goal is only possible as a unified couple. Part of that principle is that, at the most fundamental level, sex is meant to accomplish two objectives: 1) be an integral part of truly becoming one (which is the ultimate objective of marriage) and 2) be the process of replicating yourself.
I teach my children there are very effective ways to circumvent #2, but there is no way to accomplish #1 without sex - and that “full, undivided” unity (physical and emotional) cannot be accomplished if you are sharing yourself with more than one person outside of a committed relationship.
I think, in our modern world, we simply have to remove children from the equation when we discuss chastity. If sex is only (or even primarily) about kids, and not fundamentally about the joining of our entire souls (body and spirit), then we have turned a religious principle into a scientific issue - and we lose in this day and age of readily available birth control.