Sex is never 'just sex', but is an act of the whole person
One of the best explanations of why sexual intercourse is appropriate only within marriage is found in John Paul II’s Theology of the Body, the 129 lectures he delivered at his Wednesday audiences between 1979 and 1984.
Explaining sex in terms of the nature of the human person (because, really, what other approach makes sense?), he gives the context for sexual morality by first showing that there is a spousal meaning of the body – a meaning which points beyond the body to the relationship between man and woman in their full personhood. The sexual act is not merely physical, but it involves the whole person, who has a special dignity and value as a human person. But contrary to human dignity, premarital sex reduces the human person to an object of use as opposed to being an act in accordance with the person’s value. But how is this so?
The body has a spousal meaning, of which sexual intercourse is meant to be the fullest expression – a sign of a total commitment made between two persons, and made to their whole persons, not a temporary one made to their bodies alone. The only commitment total enough to render the sexual act in accordance with the dignity of human persons is the commitment of marriage – a commitment sealed with a vow. Conjugal faithfulness is a “good which can only be adequately realized in the exclusive relation between the two (that is, in the spousal relationship between one man and one woman)” (John Paul II, General Audience of August 27, 1980). If that commitment is not there, what does sex come to signify? Without the marital bond, a couple is committing a lie with their bodies – in a way, pretending that they are totally committed to each other when in reality, they have reduced each other to utilitarian dimensions. This reduction is rooted in an insufficient understanding of the meaning of a human person – one that denies the totality of his or her identity as a work of creation consisting of both a natural body and a supernatural soul.