When I was 15, a few of my friends dropped out and it mostly ended up being just me and another friend. After hanging out more just one-on-one, I developed a crush on him. He didn't know I was into guys, and I wasn't sure how he would feel about it, so I never really did anything about it.
One school holidays, his parents went overseas and he had his house to himself. He invited me over to hang out and have a sleep-over. I had no expectations anything would happen, hell I hadn't even considered it. The whole time I was there, he kept trying to flirt with me and set up situations which might lead to us doing something together, but I was such an awkward kid I hardly picked up on any of it. Looking back now, it's really kind of obvious, but at the time I just didn't realise.
After picking up on absolutely none of his hints, nothing happened that night. We watched some movies, then went to sleep in his lounge room. I was on the couch and he was on the floor right next to the couch. When we woke up, we were just laying there chatting, and then he started playing footsie with me, giving me the look. I didn't know how to react, freaked out a little, then quickly sat up and started talking about something else, acting as if nothing had happened. He looked disappointed, but that was it. I went home shortly after and then saw him again when school went back.
We still got on well, but something was a little different, as if there was a bit of tension. We never spoke about anything and I changed schools after that year, for unrelated reasons. We never kept in touch and I haven't spoken to him since that year.
It's not a regret because I could've lost my virginity five years earlier than I did, rather I regret it because it could've been with a friend who I had genuinely developed feelings for. Who knows, it could've been a one-time thing which filled us with regret, or it could have led to something a bit more substantial.
A few years later, when Facebook started getting popular, I did the same thing a lot of people did and added old school friends I hadn't thought about in a few years. I found his profile and he was engaged to a woman and seemed to be doing well. I sent him a friend request, not hoping for anything romantic but to just chat and maybe hang out again as friends. He never accepted and I don't know if he blocked me because I couldn't find his profile later. It'd be cool to catch up one day, but I doubt it would happen.