When I (male) was 13, I got my first boyfriend. A 19 year old who I lied about my age to for 95% of our relationship (douchebag move on my part I know, but horny teenage boys and all that).
An interesting thing about him was that not only was he gay but his mother was a lesbian and his uncle was gay too.
Well one day his uncle (who like my boyfriend at the time, thought I was 16 ie legal age here) was driving me home. During the ride he asked me if I "play around". I didn't. He put his hand on my knee and I froze. He still had the keys to the house he was moving out of and we ended having sex against the hallway wall. It was amazing. I'd only slept with one guy at that point but this was an experienced older man.
It happened once more after that, but in a much more organised way. Only that time the guilt started to hit me right in the middle and I wanted it to stop. The best way to do that? Tell him my real age while I'm sat, naked, on top of him of course.
He looked like I'd pulled a gun on him. He took me home, gave me a huge talk about how much trouble I could get his nephew in (now he cared about his nephew)...and 2 days later he tried to get me into bed again. I shut him down, the guilt of cheating was just too much for me. And I think that experience is why I've never cheated on anyone in the 10+ years since.
So I cheated on my boyfriend with his uncle, who was literally the same age as my father, while underage. Said boyfriend once admitted to me that he and his uncle did (totally consensual) sexual stuff together in a gay sauna too. I've always regretted not giving him a rendition of "shut your fucking face, uncle fucker" when we broke up, because when is that opportunity going to come up again? :(
Edit: I should note, although he came onto me again after finding out my real age, I felt bad for the uncle. In the car ride home after, he was almost in tears because he had been sexually abused as a kid and felt like he'd just done that someone else. Lawfully, yes. But as far as I was concerned, I made that choice, I knew exactly what I was doing even if I didn't have a real conception of just how much ruin I could bring down on someone. I was already sexually active and there was no force or grooming, just a stupid kid and a clueless adult. Luckily my actions never ruined anyone's life.