My Mom's husband works out a lot, and sometimes naked. The only reason I know is because he usually leaves the door open a little bit, and when I come home from school, I go down the hall, and look through.
I saw him masturbating a while ago. I thought he was just working out naked as usual, but then when I went to look in the crack of the door, I saw him laying on the bench jacking it. He kept sniffing his armpit when he jacked it, and after a while he came really hard. He made some growling noise and I saw him shoot his cum onto his face. He opened his mouth, and shot right into it.
He got up, and started to lift weights again, with his erection still standing up in the air. I watched for a little bit and then went back to my room. My mom came home a long time after that, and we had dinner together. I never said anything to my stepdad or my mom about watching him, but every time I get home from school, he is working out naked. I hope I get to see him jacking it again sometime.