I believe the science behind it, is that when you have an erection, the Valves switch, closing off the stream of urine, helping you retain until you get up in the morning.
When you have an erection, I bet its hard to start Peeing?
If so, then thats probably because you have to force your body to open the other valve.
Only once or twice had I gotten a hardon from having to pee badly, during the day.
Its almost a daily thing in the morning though. Soon as I pee, bye bye boner.