What's your biggest ever sexual regret?

Since we are a couple, there are some.

Husband when single, regrets not having sex with a young student lady while he was working as an instructor for plant designing. She was 18 and looking very innocent, saying she was untouched. Well, but two weeks after the negative he heard she was then banging some random dude of the same class.

Wife when single, regrets not having accepted an invitation to go to a swingers party with a horny guy she was fucking around at the time she was 20. She went to some parties like that already as a hotwife, but she feels with would be wild doing it earlier.

As a couple, we regret some things we did and some other things that we didn't. The biggest one was probably wife started to cheat before we had an open talk about our desires related to sexual matters.