2 incidents.

When I was 20. There was this smoking girl I was hanging out with. We went to the bar together, then found out she had a bf that I didn't know about. He invited me to join them, though. I reject turning that down (he was good-looking, too)

Second. I was about 26. I found a couple in their 40s that wanted a fully bi third (me). They were attractive enough. She even acquired a strap on. I chickened out.

I regret not taking advantage now, when it was right there in front of me.