Sex and sexuality are an integral part of human life and vital for the survival of the human race, but sexual freedoms in many countries have yet to be enshrined as constitutional rights. Focusing primarily on Japan, Sex, Sexuality, and the Constitution critically reconsiders the relationship between individual sexual freedoms and a constitutionally entrenched right to sexual autonomy. Shigenori Matsui explores the extent to which governments should be allowed to restrict or influence sexual autonomy to support desired population policy outcomes. Should the constitution encompass the following rights: an individual’s right to decide or change sexual or gender identity; to have sex or to refuse to have sex; to have children, through natural birth or through access to medically assisted reproduction; or to not have children, through access to abortion? This rigorously detailed legal analysis has implications for government policy in all countries facing similar population and constitutional rights challenges.