The debate over whether single sex spaces should be based on biological sex or gender identity has grown out of wider legal, social, and cultural changes around sex, gender, rights, and public accommodation. Historically, spaces such as sports categories, shelters, prisons, bathrooms, and changing rooms were organized around biological sex, largely to address privacy, safety, physical difference, or administrative simplicity. Over time, evolving understandings of gender identity, alongside anti-discrimination law and transgender rights recognition, pushed institutions to reconsider how these spaces should be defined and who they are meant to protect. Today, the issue is no longer limited to one setting. In sports, it connects to classification, physical performance, and category design. In shelters, it relates to trauma-informed care, privacy, and access to protection. In prisons, it raises questions of placement, vulnerability, and institutional responsibility. Because each setting serves a different purpose, the debate is complex and cannot be understood through a single lens.