Saying that police line-ups are objective or in anyway use the scientific method even in the best case is flawed thinking. Most famously, ask Ronald Cotton and Jennifer Thompson about how accurate line-ups are, or the hundreds and thousands of other people falsely accused based solely on "eye-witness" testimony.
People's memories are faulty and malleable. Morals are similarly malleable. In order to establish a scientific method of identifying content you'd first have to get everyone to agree on the definition of lewd. The first judge or prosecutor that disagrees with that definition based on his local jurisdiction's interpretation or the church he goes, and the scientific method gets thrown out the window due to public outcry or political expediency.
Nice goal in theory and I'm sure people will keep trying but in reality we need to put more emphasis on education, outreach, and understanding than we do on creating a new set of complicated rules that people will eventually skirt anyway.