Dallas Willard:

We are sexual beings: “male and female created he them” (Gen. 1:27). This crucial passage ties sexuality to our creation in the image of God. It is a part of our power with which to serve him. In sexuality the intermingling of person, the knowing and being known that is characteristic of God’s basic nature, is provided in a special form for embodied personality. In the full sexual union, the person is known in his or her whole body and knows the other by means of his or her whole body. The depth of involvement is so deep that there can be no such thing as “casual sex.” It is a contradiction of terms—something very well understood by the apostle Paul, who, accordingly, taught that fornication is a sin against one’s own body (1 Cor. 6:18).

—Dallas Willard, The Spirit of the Disciplines: Understanding How God Changes Lives (New York: HarperCollins, 1988), 171.