It was when I was 19 and at University. We'd had a small party at our student flat and were a bit drunk. One guy someone had invited had been a bit flirty with me all night. At the end of the night, he asked if he could sleep over on our sofa, since he was a long way from home and it was late. About half an hour after everyone had gone to bed, he crept into my room and whispered “are you awake”. I could see in the shadows that he was just in his boxer shorts. He got into bed with me and we started kissing and touching (running my hands over his chest). Eventually, he guided my head down to his groin, whispering “I'd love you to suck me”.
He slid down his boxers, so my face was right up close to his very hard penis - quite thick, his foreskin already drawn back, surrounded by a good thatch of pubic hair. I remember being excited and nervous about taking a penis in my mouth for the first time - so I began by licking the head of his penis. Obviously, he wanted a bit more than his, and after a while gently pushed down my head, so I took his shaft deeper inside my mouth and began sucking.
He must have known I was inexperienced, because after a while he said, “have you done this before?”. I took his cock out of my mouth and said, “no” and he said, “you're doing good, carry on!”
It felt like I was going to gag at first, but got easier as my spit lubricated his shaft, and the in-out motion I was doing felt better. His breathing started to get heavier, and I began to taste his salty pre-cum. Then he gasped, “I'm going to cum!” and immediately afterwards ejaculated. I was surprised at how sudden it was - in a split second my mouth filled with his semen. It seemed like a lot - and all I could think to do was swallow.
He didn't reciprocate - instead he masturbated me, till I came on my belly. In hindsight, I think he was probably inexperienced too. After that, he said he was going back to sleep on the sofa, so my flatmates didn't find out. So I went to sleep with the taste of his semen in my mouth.