All right, gentlemen, it's time for Legal History 101.
Periodically the United States, or its various sub-divisions have undertaken to "wipe out pornography". These fits of nonsense have usually coincided with the ends of wars, for some strange reason. In post-Civil War period (1873) the U.S. Post Office passed a regulation banning the mailing of "obscene" materials. At first this only applied to the mailing of birth control information and "devices" (condoms). In the post-World War One period a postmaster general appointed a self-righteous preacher to make up a list of other "obscene" things that couldn't be mailed. His list included and "depiction of the adult human body bearing public hair". The only exceptions were medical texts and nudist publications.
Strangely, he banned only public hair, not depictions of genitalia.
This ban was still in effect in the early 1950s when Playboy started publishing. Rather than fight it, Hefner protected his 2nd class mailing privileges by using shaven or air-brushed models. In the late 1950s Milton Luros arranged with the major American nudist association to publish their magazines, and started his own Jaybird series which did show pubic hair. A U.S. district attorney sued him for mailing "obscene" material and in 1967 the SCOTUS ruled that if congress was going to ban obscene material, they needed to specify in the law what was obscene. They couldn't leave it up to some preacher jacking off in his own office. As you can guess, that is a can of worms congress has not seen fit to open.