The prossie didn't couldn't tell the differance between spunk and urine, yeah right
Providing you are rock hard isn't the uretha clamped shut, so you phyically can't piss.
After long sessions of sex I sometimes find I'm busting for a piss when I start going soft, usually follows a particularly intense cum and the piss has the force of a karcher jet washer as it blasts the enamel off the prossies toilet bowl.