Its happened to me on several occasions. Some really an accident, some not so much of an accident. A few years ago, I walked out the shower, was staying with my parents then. There was nobody home, and we left the front door unlocked most the time. My moms friend was standing in the kitchen, as I walked out, I was surprised to see her. Ended up dropping my phone and speaker by mistake, next thing my towel was on the floor. She just stared at my cock. I picked up my towel and apologised. Got changed, explained that my mom had gone out. She had my number for convenience I guess. She later texted me saying it was the biggest she had ever seen, and asked how big it would grow. She was after me for 2 months till I gave in. I had broken up with an ex so wasn't really looking at the time.
So 2 months later, she happened to show up, and my parents were out. I was really horny that day. I told her she could wait for them. I was just watching some tv. I knew she had porn on her phone. I asked if I could check it out maybe copy some to my phone. So we sat next to each other, she showed me her favourites(big dick videos of course). My cock started to grow down my leg. It became increasingly visible. She started rubbing along the top of my cock shaft. It got really tight in my jeans. She asked if sje could take it out. At that point I was happy she asked. She unzips my pants and tries to pull it out. It's wedged in tight hugging my thigh. I lifted my ass off the diwan and slid my pants all the way off. She doesn't even hesitate to grab it. Stroking with one hand, longer and longer strokes as it reaches maximum potential, and then her second hand is on my cock. Stroking together till she can't resist and put her lips around the top of my cock head. She is licking and sucking. Pushing her head down and stretching her mouth wider. I could feel her teeth starting to scrape against my glans. I didn't really care, I was looking forward to cumming. Eventually she got her mouth and throat half way down my thick cock, stroking faster and faster eager for my cum it seems, or just really excited. I expected she could feel my cock grow even longer and especially thicker just before I am about tp ejaculate. I didn't warn her, got her by surprise. Shot right down the back of her throat, she coughed, but kept going till she couldn't swallow or hold them cum in her mouth any longer. The gooey mess just oozed from her lips, down my shaft, over her hands, and settled on my balls and pubic bone. As soon as the throbbing subdued and the cum stopped flowing I got up to shower and get the sticky mess of me. Walking 10meters to the shower, with my cock slapping on my thigh and cum dripping along the floor. I had to wash up quickly before my parents got back so I could clean up. She went to wash her hands and face as soon as I was done.
She pursued me to have sex over some more time. But I wasn't really interested. I was pissed with girls at the time, and just did things that suited me. I can see that was selfish.. But at the time I felt like I needed that.