Aretino and Giulio Romano
Pietro Aretino's major work of pornography was the "Sei Giorni," an imitation of Lucian. Up to that point, pornography (whatever the term that might have been used) had been considered a literary matter only.
The medieval grotesques did not enter into consideration (though strictly speaking they should have).
The crucial step was the collaboration between Aretino (sonnets) and Giulio Romano (engravings) of 1525/6, sometimes termed "I modi." Only at that point did pornography attain a potential status as a category in the visual arts--a status that is now pivotal, as no one is concerned these days with banning D.H. Lawrence or Henry Miller. Words, it seems, can't hurt us, but pictures can.
There is a well-illustrated monograph on the Aretino-Giulio collaboration by Lynn Lawner, I modi, the Sixteen Pleasures: An Erotic Album of the Italian Renaissance. The definitive work is Betty Talvacchia, Taking Positions: On the Erotic in Renaissance Culture (Princeton U Press, 1999); this book is now quite expensive.