Many years ago we rented a house for a holiday weekend with three other couples. After celebrating late, we'd all had a lot to drink. My wife and one of the other men both went to their respective beds. The other three women - after a lot of persuasion and wine - did a topless dance for the rest of us. Sadly nothing more than topless but we didn't complain!
Eventually, all but the woman who's husband was already asleep went to bed. She was tired but I begged her to let me see her naked. She finally said she would if I did too. We both undressed - I was as hard as I could be. I really wanted to touch her and she allowed me to put a finger inside her - which I licked after I pulled it out! Then she said "that's enough" and went to join her husband.
The next morning, my wife woke up early - I was feeling hungover and had a lie in. Big mistake. I'd forgotten my underpants were on the floor in the front room and she found them! I tried to convince her that nothing had happened but know that she didn't really believe me. Luckily she had no idea who the other woman was.
My wife has never mentioned it since - and my friend has no idea that I fingered his wife. But I often think about those few brief moments!