One of many things that happened
One day years ago, I came home to find my oldest step daughter on her own. She was in my room using the computer for homework.
When I got there I had to use the bathroom quickly and then I went into my room to relax a bit. As we chatted I noticed that she was occasionally glancing at my crotch. Since we had a very open relationship I admitted that I was having some very erotic thoughts on the way home. I mentioned I had to use the bathroom badly and had heard that a man can't urinate with an erection, so to help me make it home I was thinking about sexy things to see if it worked. She laughed at me and my silliness and stated it couldn't be true because even though I had gone to the bathroom I was still...well it was still obviously erect.
After a few more minutes of laying on the bed still hard, I stated that it also had other problems. She asked 'what?' Then I explained I was now very horny. So do something about it was her response.
Next thing I knew my cock was out of my pants and I was stroking away. She kept working on the computer but I noticed she wasn't really doing much. She was however frequently looking over at me as I masturbated just a few feet from her. She then stopped working completely and turned towards me as my hand stated going faster. I shot stream after stream into the air and onto my stomach. As I recovered, I looked at her and she smiled then went back to her work.