Because it is a kind of social control. People really need to read more Foucault.

Short version: https://plato.stanford.edu/entries/foucault/#HistModeSexu On Foucault’s account, modern control of sexuality parallels modern control of criminality by making sex (like crime) an object of allegedly scientific disciplines, which simultaneously offer knowledge and domination of their objects. However, it becomes apparent that there is a further dimension in the power associated with the sciences of sexuality. Not only is there control exercised via other people’s knowledge of individuals such as doctors’ knowledge, for example; there is also control via individuals’ knowledge of themselves. Individuals internalize the norms laid down by the sciences of sexuality and monitor themselves in an effort to conform to these norms. Thus, they are controlled not only as objects of disciplines but also as self-scrutinizing and self-forming subjects.