Defeminization and masculinization are the differentiating processes that a fetus goes through to become male. From this perspective, the female is the default path for a developing human being in that gene actions that are eliminated and that are necessary for formation of male genitalia lead to the development of external female genitalia.
Biologically, this perspective is supported by the fact that there are neither female genes nor female hormones that correspond to the hormones active in males only. Estrogen, for instance, is present in both the male and female fetus.
Read more about this topic: Sexual Differentiation