Nudie magazines and letters were already a thing of the day. There were even mail clubs for just that thing. I live in a State where western brothels flourished. Our founding fathers had a few that weren’t exactly chaste. We built this nation with sex as a mainstay, so something else has to be causing the moral decline.
The “Progressive Movement”, which had a major surge in 1913, including its own political party of which the likes of Roosevelt belonged, has caused the moral decline far more than nudie pictures.
Well, there were “art studies” magazines, and sex pamphlets, rather commonly in the 1920s/30s. Ads to send away for Frech postcards, to nudie reels. And even further underground, things like Tijuana Bibles, which were really vile and depraved items, and likely get one arrested if caught trying to sell them throughout most of America.
But you don’t see that specific promotion of a lifestyle/mindset until Hefner in the 1950s. I believe what he did to the culture was actually more damaging and more insidious. Not that the ‘nude’ pictures and pin-ups in his issues of the 50s/60s were all that egregious. It was the debauched, no-responsibility worldview being sold. Trashier imitators gradually surpassed his product, of course, especially by the 1970s. Probably made him and his ethos seem reserved and quaint in comparison, which has probably fueled some of the retro-nostalgia associated with him and his magazine.
But yes, I do figure Hefner prominently in the course of America’s moral decline. And the pics of nekkid gals weren’t really the primary culprit.