J. Budziszewski (as usual) has a very thoughtful piece in Touchstone entitled Designed for Sex: What We Lose When We Forget What Sex Is For. Here is a summary of the four themes he sets forth:

The first is that we ought to respect the principles of our sexual design. Just as those ways of living that flout the bodily aspects of our design sicken and kill us, so those ways of living that flout the emotional, intellectual, and spiritual aspects of our design ruin us and empty life of meaning.

The second theme is that the human sexual powers have a purpose. As the purpose of the visual powers is to see and the purpose of the ingestive powers is to take in nourishment, so the purpose of the sexual powers is to procreate. This purpose is not in the eye of the beholder; apart from this purpose, we would have no way to explain why we have them. Moreover, if we try to make use of the sexual powers in ways that thwart and violate this purpose, we thwart and violate ourselves.

The third theme is that the human design for procreation requires marital and family life. For guppies, it doesn’t; they manage to procreate without them. For us, however, it does. To put this another way, we are made with a view to marriage and family, and fitness for them is one of our design criteria. No one invented them, no one is indifferent to them, and there was never a time in human history when they did not exist.

Even when disordered, they persist. Spouses and family members who are divided by disaster commonly undertake Herculean efforts to reunite with each other. Marriage and family are not merely apparent goods but real ones, and the rules and habits necessary to their flourishing belong to the natural law.

The final theme is that the spousal bond has its own structure, which both nourishes and is nourished by these institutions. Because it has its own structure, it has its own principles. Among these principles are the following: Happiness cannot be heightened by sexually using the Other; conjugal joy requires a mutual and total gift of Self. Feelings of union are no substitute for union; their purpose is to encourage the reality of which they are merely a foretaste. The procreative and unitive meanings of sexuality are joined by nature; they cannot be severed without distorting or diminishing them both.

Read the whole thing. Also worth reading, and also from Touchstone, is Bodies of Evidence: The Real Meaning of Sex Is Right in Front of Our Eyes by Frederica Mathewes-Green.