About 10 years ago, my wife (now ex-wife) and I were on a snow ski vacation with several of her college sorority girlfriends and their new husbands, and my wife and I had to share a bathroom with another couple. On one of the nights, I was in the shower and my wife was also in the bathroom putting on her makeup. We were running late for dinner, so my wife's girlfriend asked if she could come in and start on her makeup while I was showering.
The bathroom was very small and with me being 6'5", I could easily see over the shower curtain at my wife and her friend. The shower curtain was not transparent, but you could see through it if you looked hard enough. I started to slowly stroke my cock and immediately my wife's girlfriend started watching me. She would not look me in the eyes, but I continued to jack off while I watched her watch me. My wife was clueless.
True story.