Why would this mostly only affect people who happen to have genitalia inside their bodies?

Mass sociogenic illness is one of the oldest documented patterns in medicine. Historians have traced examples back to the dancing plagues of medieval Europe, when whole villages spontaneously began dancing to the point of collapse and death. Salem in 1692. The nuns of Loudun in the seventeenth century. A group of teenage girls at a high school in LeRoy, New York, in 2011, who developed Tourette-like tics with no organic cause. In every case, a group of people, almost always young, almost always female, and almost always emotionally connected to each other in some way, develop the same physical symptoms in rapid succession, and the symptoms cannot be explained by any physical illness the medical system knows how to diagnose.