A Free Ride (1915) is probably the oldest American porn film and it contains both male and female peeing. Link to Wikimedia Commons (the film is public domain): https://commons.wikimedia.org/wiki/File:A_Free_Ride_(1915).ogv
The male pee scene starts around 2:05 minutes and I noticed that the man has a strange way of holding his penis when doing the deed. His hand is sort of the other way around with the thumb and forefinger positioned on the base of the penis instead of the head (the way I