Hen na Chishiki ni Kuwashii Kanojo Takayukashiki Souko-san - Vol. 1 Ch. 13 - Desire to Urinate

The reason why it is blocked is because urine kill sperm as they share the same path. How it evolved that way is via natural selection as only those who are blocked managed to reproduce as those that aren't could not as the sperm is usually killed even before reaching the egg not mention the damage urine have to the egg cell.
 
@Guspaz Like @TrueGoddessReincarnation said, peeing with a boner is possible. As far as I'm concerned it's only when you're in your climax or right after your ejaculation that you can't pee, with that in mind to me saying that you can't pee while you have a boner is like the old myth of getting a bloody nose when looking at something sexual enticing.
 

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