My older brother and I were both home from college for the holidays. He was planning to switch to my room because my grandparents would be arriving that evening and were taking his room. He had already moved some stuff in to free up a drawer for them, but otherwise still had use of his room.
I was stroking off in the privacy of my bedroom when I heard his and one of his (very hot) friend's voices in the hallway and I assumed they were heading to his room to hang out. Much to my surprise, my door flung open and they both barged in. There I was on the bed, facing the door...hard dick in hand. Because my brother and I shared a bathroom on our own floor, so it was completely normal to see each others' dicks. This was, however, the first and only time he saw me with an erection. It was the first time his friend saw my dick at all.
He said something like "oops" and laughed while I said something like "you could knock." He said he just needed to change into workout clothes, so he quickly changed and they left. I didn't have anything handy to cover up with, so my erection remained exposed. Getting up or rolling over would have just been more awkward. I was so turned on to be exposed that I was throbbing and oozing pre-cum all over the place. They quickly left and said something like "have fun," again laughing, while I said something like "next time, knock." As soon as they left, I barely had to touch it to trigger a massive explosion.