Two or three times a year, you and your wife and her sister and her sister's husband go away on vacations together. Your wife and her sister always have a LOT of sex on these vacations -- always involving cuckolding the two of you. It was really awkward for you and him the first few times, but you're basically used to it now, and you even sometimes talk about it with him -- about how you both took a while to learn to deal with it emotionally and how you now have gotten to the point where you really hope that the two of them are having a good time.
@myheartinherhands
She almost never tells you who she's going out with or where she'll be going on her date, but you do know that many of your friends have taken her out at least once, and most of them several times.
And the truth is -- it's how your wife wants to spend those "special" nights too -- to make it as clear as possible to you that you are no longer the most important man in her life.