I had a fwb a few years ago. We really liked each other but didn't want to breakup our marriages. I attended her funeral yesterday. Her family loved her as I did but couldn't say so.
My homeroom teacher when I was 17 and she was about 30. It became a once a week thing the last half of my Senior year of HS. She was married and her husband treated her badly.
Sex with a close female friend. It was just one of those things where we probably should’ve been friends only, because it kinda made things permanently awkward, but oh well.
All fantasies are "Lustful," thus, they are wrong. So my biggest transgression is that sin on most occasions. Any Lustful thoughts of a taken/married partner is considered "Adultry," that would be the next sin.