Real love moves you to give yourself fully to a particular man or woman. Lust works in the opposite direction. It wants to get a fulfilling, self-maximizing experience from the person. In the biblical view, the purpose of sex is not personal self-expression (in order to be happy) but personal self-donation that brings permanent unity and life (in imitation of Christ, Ephesians 5:22ff.). Sex without the giving of oneself is a monstrosity, akin to a body walking around without a head. Lust and sex without marriage is like tasting food without swallowing and digesting it.

Tim & Kathy Keller