|Not really me - some younger guy.|
If you open the fly, the leg parts of the shorts sort of stick out and you've got to try to hold them back so that you don't accidently pee on them. I remember that, when I was a boy and shorts were the norm, we boys used to pull up one leg and pee through there. This worked well. As summer school shorts didn't have a fly, this was a common proceedure used by all. I've tried this method and it certainly works better than using the fly. Maybe someone needs to invent a device so that it is easier to pee through the fly when wearing shorts.
Bicycle clips for shorts would be one solution, though the down side is that you'd have to carry them around.
Maybe a tube thing would work too - a thing like a hose that fits onto your dong.
Another option would be to sit down, but you couldn't do that at a urinal. Very tight shorts might be another solution.
I guess you could use one of those bottle things they have in hospitals.