When my brother was in college and I was still in high school, he would usually get home about a half hour before I did because he only had a few classes per day. One day when I came home his girlfriend's car was in the driveway, and when I went up to my room his door was shut and I could hear her moaning and him pumping into her.
Let me just say, I have seen his penis before and I know it is BIG. I've seen him soft many times either changing clothes or walking back naked from the shower, but the two times I have seen him erect (one he was sleeping naked and had an erection in the morning, the other I walked in on him masturbating in the bathroom), I saw he was huge. Probably pushing nine inches and thick, so I knew she was being rocked hard. Just thinking about her being filled by that big penis was getting me horny, and I knew I needed to cum. I closed my door and stripped down naked and started to rub myself, but that wasn't doing it. I looked around and found one of those big novelty pens I've had for years. I put a little lotion on it and slowly slid it inside me, filling my vagina and letting it get used to the fullness before I started sliding it inside and back out.
I closed my eyes and listened to her panting and moaning while I fucked myself with that piece of purple plastic. I wanted some leverage so I spun around and put my feet on the wall, lifting my butt and back off the bed and pounded my poor little pussy, imagining it was his huge hunk of meat between my legs stretching and abusing my vagina, exploding with a fantastic orgasm that made my eyes water. I relaxed and laid there for a few seconds but I wasn't satisfied, so I got out of bed and stood in front of my mirror, putting my foot on the dresser and started sliding it inside me again, watching my every move. It took a couple more minutes, but I watched my toes lift and spread off the dresser when my second orgasm hit me, and I had to stumble over to my bed and sit down to catch my breath.
It wasn't until I was sitting there that I noticed everything had gone quiet, and I was kind of embarrassed because I wasn't sure if they heard me masturbating or not. I threw on my sweats and went downstairs and they were laying together on the couch, him just in a pair of gym shorts, not hiding his after-sex engorged penis. We bullshitted for a little while but nothing was said about either of our actions then or later that night.
That day was the first time I really masturbated thinking about his cock, but it certainly wasn't the last.