In the summer of '85' I was a horny 35 years old married with two small children. We travelled to my brother-in-laws house in jersey for a one week vacation. When we arrived my wife and I were given their master bedroom upstairs and our kids slept with their kids. My wife hung our clothes in their closet in the bedroom. The first morning when I got up my wife was already downstairs talking with her sister so I went to the closet to get my clothes before I dressed and showered.
In the closet I looked above the clothes on a shelf and saw a box with 'prelude vibrator' on the side of it. I took it down and opened the box to see who was using it. That was easy because it had the 'bulb style' tip on it. I knew it was her sister's vibe because that's the attachment most women use on themselves. Also in the box was the 'cup style' attachment. I checked the stairway to see if it was safe and them proceeded to put the cup attachment on the vibe and plug it in. It had two speeds so I clicked it on low and put the cup over the end of my already rock hard cock. The vibrations were absolutely driving me crazy, but I knew I had to hurry so I clicked it to high and it buzzed wildly on the end on my cock sending me into an orgasm in about one minute. I was still shaking as I wiped it off and put it back as I had found it. I used it a couple more times that week without anyone's knowledge. I sometimes think about my sister-in-law using that vibe on herself not knowing that I had used it on my cock too.