Why not say that the meaning and purpose of the sexual powers is pleasure? Certainly sex is pleasurable, but there is nothing distinctive about that. In various ways and degrees, the exercise of every voluntary power is pleasurable. It is pleasurable to eat, pleasurable to breath, even pleasurable to flex the muscles of the leg. The problem is that eating is pleasurable even if I am eating too much, breathing is pleasurable even if I am sniffing glue, flexing the muscles of the leg is pleasurable even if I am kicking the dog. For a criterion of when it is good to enjoy each pleasure, one must look beyond the fact that it is a pleasure. Consider an analogy between sex and eating. The purpose of eating is to take in nutrition, but eating is pleasurable, so suppose that we were to say that the purpose of eating is pleasure, too. Then it would seem that any way of eating that gives pleasure is good, whether it is suitable for nutrition or not. Certain ancient Romans are said to have thought this way. To prolong the pleasure of their feasts, they purged between courses. I hope it is not difficult to recognize that such behavior is disordered. The more general point I am trying to make is that although we find pleasure in exercising our sexual powers, pleasure is not their purpose; it only provides a motive for using these powers, and a dangerous one, too, which may at times conflict with their true purposes and steer us wrong. Besides, to think of pleasure as the purpose of intercourse is to treat our bodies merely as tools for sending agreeable sensations to our minds. They are of inestimably greater dignity than that, for they are part of what we are.
J. Budziszewski, On the Meaning of Sex
human-body, natural-law, pleasure, pleasures, sex, sexuality