advances were not the only necessary condition for SRS to arise. Technological capacity had to be paired with a hospitable theory of sexuality. And just such a theory was in the air in the late nineteenth and early twentieth centuries: the theory of the universal constitutional bisexuality of humanity (i.e., the idea that human sexual differentiation is nonbinary in nature). The germ of this idea can be traced back to Charles Darwin, who set the stage for a “new genderless human nature,”10 and it