You're right, it isn't a trivial matter. However, neither has it been well explained. For example, how exactly has porn been ruled out? Porn also grew steadily since mid 20th C...
Correlation is not causation. Computing performance has also increased significantly since the mid 20th. Should we blame Intel for the drop in T levels? Where the hell do you get the idea that porn reduces testosterone? Research shows that watching porn increases testosterone levels in men https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2799222/