I (now a mid 20M) would have the nastiest sex of my life with my ex (now a mid 20F) while dating my girlfriend at the time as 19 year olds
Don’t take this as an endorsement for infidelity, I was 19 and foolish and would never do this now, but it was fun while it lasted.
I dated a girl all through high school, we can call her Allie. Allie was about 5’8”, medium length dark brown hair, curvy, and very innocent looking. I was the only guy she had ever been with. Then, she cheated on me in college and we broke up. After some time, I started dating another girl I had met, we can call her Katie. I would run into Allie almost every day in college, as our classes were near each other and my fraternity was often involved in socials with her sorority. One day at a social, she pulled me into the hallway so we were alone, sat me down and said she wanted to apologize for everything. I told her I forgave her and we could move past it. We stood up and hugged and went back into the social.