Most people think of "chastity" as something negative - NOT doing something (bad). And that is certainly a dimension of chastity. Chastity means regulating one's sexual capacity in the right way, by keeping it in the context of marriage, and doing sex the way it is meant to be done, i.e., in a way that produces true marital union, a true marital act.
But, apart from NOT doing things that are wrong, chastity involves -- because it is a real virtue -- doing something right. In fact, the word "virtue" in Greek is the same as the word for "excellence." So "virtuous, chaste sex" is "excellent marital sex."
So what is this positive dimension?
Perfect chastity means especially the perfecting of my love for my spouse and the complete orientation of my sexual activity (thoughts, words, actions) toward that love of her.
This involves seeing my sexual activity in its broadest framework, the framework of my whole life of love with and for my spouse. And this includes not only our affectionate dealings with each other, and our marital sex, but also the fruitfulness of that love, which takes form especially in the children it has brought into life and educated. Men especially tend to look at sex as a particular act, and they have trouble looking beyond that particular act. Often, they focus on only its physical aspects, instead of seeing that individual act of marital lovemaking as embodying the entire life he shares with his wife, as expressing the fullness of his marital relationship in the course of their entire life together.
Think about having a meal with some friends. Now imagine a person who focuses his attention solely on the food, and doesn't pay any attention to the friends with whom he is having the meal. Wouldn't that be "off" - somehow out of whack?
Even if, let's say, he were to buy the meal for everyone and he also very sincerely hoped that they were enjoying their food as much as he was enjoying his, but he still spends the entire meal simply focusing on, being absorbed in, the delight of his own food, wouldn't that be weird? Don't we see that a person sharing a meal with friends has to be focused on those friends, not just the good food. In fact, I think we could say that he should be primarily focused on his friends, while at the same time enjoying the food as part of this "sharing" of himself with our friends. The primary focus should be on them, because people are much more important than food. We need food, of course, but that need doesn't make the food "better than" or "higher than" or "more important than" the sharing the meal with our friends.
Well, the same thing is true of marital sex. (Actually, it's even truer -- because we don't share the same food when we eat with friends, while we do share our own bodies in marital lovemaking.) We should certainly enjoy the physical delights of sexual activity, just as we should enjoy the delights of good food. But what matters most is the person -- our sharing of ourselves in a deep and intimate way with this person to whom we have given ourselves completely for life, and from whom we receive the gift of her life.