This is the only modern recorded case of this happening in about 100 years, so considering all the other scary shit to avoid in the Amazon, this one should be pretty low on your list. I know people always make a big issue out of something getting in their penis, but statistically it's a lot less dangerous than malaria, yellow fever, or leishmaniasis.
The motives of the fish are to find nice catfish gills and eat it's blood. It detects them because fish take care of metabolic waste( pee) through their gills. Unfortunately for all party's it seeks out dick when your peeing in the water.