Taken from S3E1 of Arrested Development, "watching the Reno sunset" means a man waiting for his erection to subside before moving to avoid drawing attention to his boner.
Friend 1: I was so embarassed after my date.
Friend 2: Why?
Friend 1: I got a boner, and didn't have time to watch the reno sunset before she noticed.
Friend 2: Why?
Friend 1: I got a boner, and didn't have time to watch the reno sunset before she noticed.
by fishmans March 25, 2020