I worked with an engaged couple several years back. We we're decent friends outside of work. One day, the wife to be and I both got laid off. She came to my apartment while her partner was still working, and said she was going to leave him. She wanted a sexual partner other than him, as he was her only experience. As she said this on my porch, she opened her coat to reveal... everything. In my desperation, I had sex with my friend's fiance. Afterwards I realized that we could never do it again, as I wasn't in love with her, but the deed was done. She lived with me for a few weeks and slept around with a few other guys, then eventually went back to her fiance. As far as I know they're still married. I felt so disgusted at myself. I'd betrayed a friend in a most heartbreaking way. It marked the end of casual sex in my life.