TL:DR, summer sleepover in a friend's basement.
Winter is coming, flick hard to get past this wall of text.
Actually, yeah. In HS, I wasn't out, I had a crush on a cute popular guy that hung with the skaters, he was hot enough that he could and did sleep with pretty much any girl he laid his eyes on. We sorta became friends over the course of the school year (don't remember exactly how we started talking), I knew nothing would ever happen but it felt nice that a guy I liked wanted to hang out, so it was bliss. I'm pretty passable for straight, few people have ever guessed, and usually it's because after knowing me for a while they probably see I'm perpetually single. I wouldn't call it a sleepover, but there were regularly a bunch of us that would crash at each other's houses over summer break. One night, at his house, he woke me up without waking anyone else up and whispered "dude, will you blow me?" my heart was racing, I quickly said no and acted disgusted, I was seriously afraid that he somehow figured out I was gay and that our other friends in the room were pretending to sleep and that if I said yes they'd beat me up or worse... Even though I wanted to say yes more than anything, I just acted like I was annoyed and half asleep. I asked him if he wanted to talk, he said sure. So we quietly went across the basement so we wouldn't wake our two other friends but they were still only about 15ft away. My heart was racing, I was excited and horny but at the same time the question really got me scared that I almost got outed, so I pretended to be sleepy and slouch around and just chat with him. I was laying on the ground and he sort of laid back and rested the back of his head on my abdomen, it felt nice kind of like a cuddle but just more chill. After talking about whatever for a few minutes he asked me again and before I could even answer the next thing I know he slid his head over so his ear was pressing against my junk. I was rock hard and he knew it. I didn't know what to do, but since he made the first move I realized he was serious and he wasn't trying to trick me so I reached over and wowza! I'd never felt someone so hard before, and he was so much girthier than I was. I gave up trying to pretend and couldn't get it in my mouth fast enough. He did the same for me, but he mostly used his hands. As you can imagine, being the first time and all, it didn't last crazy long- after like 5 minutes he had finished, I didn't know what to expect so I was caught a little off guard, and true to the closet-case I was, I pretended to be grossed out that he didn't warn me, I went over and spit in the sink and he went upstairs. He didn't finish me, but I didn't care. I followed him up, and he didn't want to talk. He seemed like he regretted it... I tried to play it off like it was no big deal, we agreed not to tell anyone and that was that.
It was a few weeks before we even mentioned anything about it to each other, and it was an awkward conversation to say the least... But needless to say, he knew I was gay and I had a crush on him, he was curious and horny, and we had fun like that quite a few times after that. I was the only guy he messed around with, and there was no kissing or anything like that- strictly physical (although he ended up helping me finish most times as well), best guess is that he was just heteroflexible enough to mess around when he was in the mood. It was great, I wouldn't trade it for anything. I've actually never enjoyed sex with anybody nearly as much as that- I dunno if it was because I had a crush on him and thought it couldn't happen because he was straight or if it's because we were really good friends and I knew it made him feel good, but either way, it's set the bar pretty high for my other relationships.