My dad would always come home from work and go straight into the shower.
My mom always had his clothes and a towel laid out waiting for him... but she was running late and asked me to set it out for him, which I did.
When he was done in the shower, he yelled out and asked if I could bring him a towel (I forgot the towel!)
When I got to the door, he had it cracked open, and was standing behind it. When he reached out to grab the towel, his mid section came into view, just for a split second.
It was the first adult penis I had ever saw, and I remember thinking holy hell...