Love and sex provide two of the primary motives of human life; the need for intimate human contact and to propagate our species. Sex is a powerful, sometimes irrational urge or instinct, but as rational creatures our human understandings and expectations of love transcend mere sexuality. Our cultural tradition says that love ideally finds its fullest expression in marriage, it says that sex properly presupposes marriage, and that the goal or purpose of sex is to eventually conceive children. Sex also can be seen as a source of pleasure and emotional intimacy, or as a special way of communicating care and concern. These different views have profound consequences for what we take to be moral and immoral, or proper and perverse. The concept of love changes with time and culture. In ancient Greece, homoeroticism was an accepted expression of love, while marriages involved power, property, and the domestic task of procreation. During the Middle Ages, love was ideally a spiritual or religious phenomenon; only in the 17th and 18th centuries did "romantic love" emerge. It is little recognized that marriages now tend to be more concerned with love than ever before.