In the West, theorists working on sex issues seek to redefine human nature and truth in the light of sexual instinct; however a sexual theory that aims to be presented based on the Islamic outlook, primarily must be able not only to redefine man on the light of human nature, but also to indicate that the sexual instinct in man finds distinct meaning and functions in the light of human nature. In this article, with reference to some requirements of the theory of human nature in relation to the important dualities of Western humanism, some of the practical capacities of the theory of nature regarding understanding and explaining sexuality in man are revealed to pave the way for a "sexual theory" based on Islamic teachings. According to the theory of essence, it can be indicated that although gender differences occur under the influence of the environment; they are not entirely constructive, and many of them are derived from the context of reality. Moreover, according to this point of view, the use of sexuality is not only applicable in the direction of human transcendence; but also the enormous variety of deviant sexual conditions in individuals can be explained. Accordingly, "modesty" as a significant distinctive element differentiating humans from animals in the domain of sexual issues, is central to the analysis of many behaviors. Therefore, family laws are organized not in the light of the concept of justice (demarcation between individuals), but in the shadow of the concept of cordiality and mercy.