Had a similar situation once on a pool.
I got out of the
water and went to the showers, took my pants off, washed myself and then got a towel around my waist so everything from waist to knees was hidden. I went back and sit down on a bench right in front of the pool, waiting for the rest of my friends. After a few minutes I noticed that almost all people from the pool looks at me, smiling and/or laughing.
What I soon realized, is that the towel hides everything from my perspective... but all the people from the pool, looking from below, could easily see my shining genitals.