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 freedoms and constitutionally entrenched protection of sexual autonomy. As a sharply declining birthrate triggers population loss and shrinks the working population in Japan, national spending on social security is under increasing pressure to support an aging population. In response, government policy is increasingly focused on boosting the birthrate. Shigenori Matsui explores the extent to which governments should be allowed to restrict or influence sexual autonomy in pursuit of population goals to support desired population policy outcomes. Should the constitution protect the following rights: an individual's right to decide or change sexual or gender identity; to have sex, to refuse to have sex; to have a child, through natural birth or through access to medically assisted reproduction; or to not be forced to have a child, through access to abortion? This rigorously detailed legal analysis argues for sexual autonomy as a constitutional right, a position that has potential implications for government policy not only in Japan but in all countries facing similar issues.