This happened a couple of years ago. My girlfriend and her parents went away on vacation for a week and they asked if I could go by their house every couple of days to get the mail and have a look around.
One day when I went over I started to get really horny. My girlfriend had been gone a few days and I was going through some sexual withdrawal while she was gone. As I stood in her room, I started to think of all the hot times she and I have had on her bed. I went over to her dresser drawer and started to go through her panties and bras. She had so many sexy pairs that I loved to see on her. As I was doing this, I came across my favorite bra of hers. It was this silky material that was so soft and smooth.
As I touched it and thought about how hot her breasts looked in the bra, my cock started to get really hard. Being so horny, I couldn't help it, I needed to cum. I took off my pants and lay down on her bed. I wrapped the silky bra around my shaft and started to stroke my cock. The soft silkiness felt so amazing and soon I was ready to explode. Not wanting to dirty her bra, I threw it to the floor as my hot cum started to shoot out all over my stomach.
I quickly cleaned myself up, put her drawer back together and left. I never told her what I had done, and unfortunately I'll never get to do it again as eventually she outwore that bra.