"I know it when I see it" has to be up there in terms of useless thought-terminating clichés together with "no smoke without fire" and "if he wanted to he would"
The point is that there is no objective measure one can use and the definition at any given time is dependent on the cultural and societal context.
Something that might have been considered pornography 100 years ago might be considered tame today.
For those unaware, this was famously stated as a test for hardcore pornography by a Supreme Court of the US justice in 1964.