The Invincible Space Streaker (1977)

Early in the film, a group of about twelve young boys strip and go skinny dipping in shallow water. Full nudity including penises visible, also when they come out of the water.

A young boy pulls out his penis (in full frontal close up) and graphically pees on a computer and the man operating it. After the man slaps him, he remains standing for a few seconds with his penis still out in full view.