The curious mystery of the missing penises.

Last year (2015) I visited several museums/sites, where I saw sculptures of nude males, missing penises -- Florence, Venice, Croatia, Jordan, Egypt, for example. I finally got up enough courage to ask a supposedly well-informed guide why that would be, and got an answer that seemed plausible (e.g., an appendage, like an arm, that is attached later in the sculpting process -- think Venus de Milo). OK (I thought), but then why are they all missing? Michelangelo's' David, and Bernini's David both have their penises intact. albeit flacid and close to the body. So now I am wondering about all the others. Were they all erect penises that got removed at some time in history because they offended some penis-Nazi? Or maybe they all just got lost, like Venus de Milo's arms? Any insight will be appreciated.