Abstract: From a perspective of real and specific man, through the distinction and analysis of social and natural sex, distorted and normal one, the one with and without love, human lasting and development, Marx and Engels pointed out that the nature of sex lies in its sociality, the sexual behaviors are based on the willing of both two parties, and their deep significance is the realization of man’s overall development. Their ethic thought concerning sex is inspiring to the construction of net sex ethics. According to them, the sexual activities on net are social activities in nature. And the aim of such construction is to guarantee the sexual right, to pursue the unification between sexual freedom and moral and hence to realize the overall development of man. The key to the construction lies in the elimination of distorted sex and advocation of normal sex.