I'll start.
I've been with my wife for nearly 10 years now. I love her deeply and I think she is one of the most beautiful women to ever walk this earth. I genuinely don't know what I would do without her.
Things haven't always been great though...
About 5 years ago I found out that my wife cheated on me. It wasn't sex or anything, and she claims that she was coerced into it, but I found out that she blew a guy that she worked with about 2 weeks after we got engaged. Things were really rough between us after finding out, and we almost ended up getting a divorce over it. However, we went to counseling, read a few couples books, and in the end turned out to have a better relationship than what we had before.
But...
I still wanted some kind of revenge.
I have always found my mother-in-law attractive. She is 48 (had my wife when she was 19), tall, blonde, skinny, has thicker thighs, a great ass, and smaller breasts. We'd always jokingly flirted back and forth, and I had always understood it as it was just her being playful. As it turns out, she was more than willing to act on those comments.
One day after she came over shortly after my wife and I were briefly separated, we just started talking and I just started complimenting her on how beautiful she was, how much she deserved a better husband than my father-in-law, and how any other man could make her happy in my mind's eye.
One thing led to another, and next thing I know she's making out with me in her bra and panties on my couch, while her daughter was crying over being briefly separated from me on her couch at her place!
Poetic justice!
Anyways, I've been having a relatively steady affair with my mother-in-law for the past 5 years. We meet up at least once a month, but try to shoot for at least two or three times a month.
Love to hear your stories in the comments!