You forgot another variable, an important one.

“You” are not “You”. Most (all?) atoms inside you are different than they once were. You are cycling through them as your body breaks down and repairs itself. Brain, teeth and eyes are about the only stable organs. The rest cycles, even your bones. And, yes, your penis.

The skin cells covering your penis, just like the rest of your body, last about a month. If you masturbate regularly, maybe faster.

So that’s one penis per month worth of skin cells, just floating around your house. And then your friend comes over, asks for a glass of water. A small mote of dust sits in the glass, invisible to the naked eye. You fill it, he drinks it.

Yeah, that’s right. Your buddy just swallowed your cock. I mean, for purposes of this absurd discussion, anyway.