@frostycab Thank you for the information; I know I couldn't have been the first to notice this. I still remember the horrified look on my mother's face (and those around us) when I asked her this question at age 11, the first time I saw the fresco.
I wonder QI also pointed out that Michelangelo's "David" had a foreskin--something likely snipped when David was an infant. Various theories have abounded on this: Some say that Florentines would have been perplexed by a circumcised penis showing the glans and corona (had they never seen an erection?) Others thought this was pandering to the prevailing anti-Jewish an anti-Muslim sentiment of the time. Most, though, agree that the model for the statue was a "street kid," who might have had a more intimate relationship with Michelangelo.
There's some further amusing/unmoderated discussion of this at https://www.theguardian.com/notesandqueries/query/0,,-82066,00.html .