Hah! I got to fuck my second wife's sister! While she watched!
She had always been somewhat flirty, and they had some prior experience sharing men. They were the first to discuss it, and my wife eventually asked me if I would.
She went to a party with the wife and I one night, and copious amounts of drugs and alcohol were consumed. Flirting turned into teasing and sneaky touching, I got to rub and massage her ass (clothed), while she rubbed on my cock with her hands. She even wanted to hold it while I pissed one time, with the wife watching, and she followed me into the bathroom another time for some heavy petting. She said I could take her home that night, but I had to let my wife go to a co-worker's house to do so. I declined the offer at that time.
When it finally happened, it was quite unexpected. It may have been just a week or 2 later, she came over for dinner after work one evening, then they started to work out the logistics in the available time. Long story short, all 3 of us wound up in bed together, with me in the middle. They took turns kissing and touching me, until the sister went down for a blowjob. After making me quite hard, she mounted my cock and rode it while the wife sat on my face.
After a bit, the wife left, and it was just me and her sister in bed. I fucked her in both missionary and doggy positions, and came in her pussy. It was a lot of fun for me, and I enjoyed her intensity after the wife left us alone. Unfortunately, they developed some hard feelings for a while; I think my wife regretted her decision and was a bit jealous, but they eventually moved past their issues.
Incidentally, maybe a year or 2 later, I moved back home for a job change, and needed some temporary housing for a couple of weeks until the wife finished school where we were then living. We asked her if I could stay with her for that time frame, but after much discussion, she declined. She said she couldn't trust herself to stay out of my bed every night, and didn't want a reoccurence of negative feelings. And she was right, I would've fucked her at every opportunity!