When I was about 15 or 16 I slept over at a friend's home on the weekend. He had an older sister who was really hot. I often thought of her when I masturbated.
This night I woke up with a raging hard-on and a full bladder - a bad combination. I went downstairs to the guest bathroom to pee. At first I could not get a pee stream going and ended up standing up and going into the sink. Even when I was finished, I still had a bit of a hard-on so I thought I would jerk-off in the bathroom but then had a better idea. I went to the laundry room and found some of my friend's sister's underwear. I was sniffing them when I heard noises coming from the basement.
From the top of the stairs I could see my friend's parents making love on a pool table. His dad had the mom bent over and was taking her from behind.
I watched them with the daughter's panty over my nose and my dick in my hand. When I came I shot all over the carpet in the hall and wiped it up with the panties.
To this day it was one of the best comes I ever had.