Wife sister swap
About a month before our wedding, my wife told me her younger sister and her had made a joking deal as teenagers to let each other do their to-be-husbands one time each before their weddings. They had not talked about it for years, but she wanted to try going through with it so she could fantasize about her turn in the future. Did not hear another word, and did not want to ask how it went.
Then one night, my future sister-in-law knocks on my apartment door. We f***** all night long and on last time on the morning before she left.
A couple of years later, the night came when it was her turn. While she was out, I swung over to my now sister-in-laws place. We never told my wife about the encore.