Sex is the sign of the marriage covenant itself. And to engage in sex is to call God as witness to hold us accountable for our covenantal commitment. So regardless of whether it makes sense to our culture or to our bodies to reserve sex for marriage, and regardless of whatever lesser commitments we have made to a women, as Christians we must realize that having sex outside of marriage makes a mockery of the covenant God instituted and to which he is a witness.”
Michael Lawrence from Sex and the Single Man in Sex and the Supremacy of Christ, ed. by John Piper and Justin Taylor