Add my name to the of been there, done that, won't do it again. Years ago I determined I'd never think twice about infidelity. Absolute, hard line "no." It's wrong. Morally, ethically, socially wrong.
One was a one-night stand at a wedding. She was 'separated' at the time.
The other one became a serious relationship and I was very much in love with her. I wanted to spend the rest of my life with her. She wasn't living with her husband at the time. I think he knew I was just another of several guys she cheated with. I ended the relationship because I knew she'd never settle down and I didn't want to get any deeper into it than I already was. I stayed in love with her for many, many years afterward.
Her life became a train wreck a couple years after the break-up. Divorce, remarry, another split, lawsuits, arrests, jail time . . .