Did the codpiece come into vogue because a bunch of Italian counts were trying to conceal their fights against syphilis? An Australian doctor makes the case:
The treatment of the disease was for the most part empirical with multiple agents applied locally, which along with the bulky dressings would give large frontal bulges, impossible to hide. The problem would present the tailors with a challenge that appears to have been met by them featuring the mass with the codpiece, while also appearing to advertise the wearer’s virility. The development of the codpiece worn by powerful and prominent leaders would not only solve the problem but also start a new fashion trend for the Court followers.
The author also notes that, at least in Italian paintings of the Renaissance, codpieces were almost always shown as colored red. The coloration, he argues, was no accident—the most common ointment used in treatment combined mercuric oxide, sulphide, and cinnabar, creating a scarlet solution. Red codpieces were thus apt to conceal any drippage.