How is 3 years old "way before any sedentary lifestyle"? I consider my family moderately active, but my 3 year old is certainly familiar with the couch. In more agrarian societies, there are probably a lot fewer opportunities to sit somewhere comfortably.

And did you consider agrarian but sexually-conservative regions (maybe India or the Middle East?), or developed but sexually-liberal regions (maybe Mediterranean?), to see if those line up with your theory?