It's not because you touched your penis, it's because you're going to touch things in the world that other people will also touch and you respect other people's preference not to touch your penis bacteria because you're not a sociopath. Right?
Vs putting a hand on their bare hip, then shaking your hand.
Also, groin sweat smells different and probably has extra hormones/pheromones in it
Maybe that didn't just remove their KFC-greased, butthole-stinky finger from their nose before we shook hands, or maybe they did.
It isn't that hard to make an agar plate and put it under a lamp for a few days. Why you try a swab of crotch juice and a swab of face juice and incubate them each and see what you get?
Assuming you go to the bathroom 3-7 times a day, that's that many opportunities to wash off the dirt you were digging in, trash you picked up off the street or someone else's hand you shook.
I think I spotted the flaw in your line of thinking.