Boys wearing gym shorts be careful
When I was in high school I always tried to be careful and not let this happen. If you aren't careful your genitals are visible and you won't even know it.
I was once in the cafeteria and the gym was upstairs. Walking down the stairs with several girls in front of me I noticed this boy with his genitals clearly visible. I'm sure that any girl who wanted to see them or many who did not got their eyes full of this careless boy.
Some boys don't care of course but if you do be careful.