Swim shorts are designed in such a way that when you’re out of the water, your undercarriage dries as fast as possible. Cotton shorts soaked in water hold moisture against your privates. As it is, moisture tends to accumulate in that area because of sweat and lack of ventilation. Storing even more moisture against your crotch is a ticket to rashes, fungal infections, irritation, etc.
I realize this is a zombie, but, at the risk of over-sharing, I had a high-school roommate who at age 15 had nuts that hung down nearly to his knees. Really, no exaggeration. He was a great piano player too, though I doubt there’s any correlation there.