We used to live together four of us, but one day I stayed alone with my friend's girlfriend.
For about one year during university times we lived two couples in a one room flat for economy reasons. It consisted of really a small room plus bathroom with toilet, but mainly we just were there during nights after spending whole days in school and evenings in some activities with other mates. We were good friends, very open in this simple environment, there was a table with four chairs and just two single beds divided with small library, so if we sat on them, of course we could see each other. We only had sex in the nights, usually all of us at the same time, because when one couple started, the other one joined, sometimes even more than once. But we didn’t watch each other nor were swinging, the only kinky part was being in the same room, getting naked under the blanket and having sex. When me or the other guy wasn’t present, for we sometimes travelled somewhere with parents and siblings, girls said they usually quietly and secretly masturbated when listening to the moans of the other couple. When they first told us we were really surprised, and to know what it was like they encouraged us boys to do the same in such situation. But the other guy actually never stayed when she was out, and I tried it only once - while lying on the side I slowly and absolutely quietly touched myself one morning when they were making love. I was really very arroused by the noises, but at some point they just finished too early for me. There was full daylight already, so I couldn’t really have imagined to cum afterwords. I carried on when they went to the bathroom together, but then again this girl appeared just after her turn in the shower, so I gave up. She was curious and asked me whether I was jerking off to them, what I admitted with no embarrasment, but I told her I usually do it faster at the end and even if I would do so I would eventually cum onto my hand, so they would figure out me cleaning and so on. It was definitely not the same like when they girls just touched themselves and could orgasm in any position. She told me they were quite noisy themselves so they wouldn’t hear me at all.
Once I stayed alone with this other girl, our partners warned us not to have sex together, we had a very funny discussion one day before and then all of us had sex in that tiny room. Next day I was in a movies and watched quite erotic film there, so I was in the mood when returning home late that evening. I found her still studying sitting by the table, wearing just the night shirt - the same all of us had from some summer camp, it was a T-shirt long almost to the knees and we used it instead of pyjamas wearing nothing under it. It was quite sexy and when I opened the door to the flat I saw a bit under that shirt of hers as she didn’t realize it was lifted up little bit. I had a quick shower, and then wearing the same kind of cloth I sat by the table too and was having one more cigarette or drink. I was sitting on my crossed legs so I only had to pull the front part of the shirt under me just to cover. Anyway, she asked me about the film, so I told her and started to have the semi-erection what was totally visible. I had it in my mind for a while, so finally I asked if she wouldn’t mind if I jacked off when we would go to beds, telling her she might eventually join. She looked between my legs and answered that I obviously already started, and as I tried to cover with my hand I just reached full erection completely against my wishes, and it accidentially lifted my shirt up. She told me she wouldn’t go to sleep that soon, because she needed to read some more from that book, but she could have a break - she stood up and turned the main light off. So right there I just touched myself and so did she, we inserted our hands down under the shirts and started to masturbate in front of each other. Later we just rolled it up, so we could see each other completely. After she came I ejaculated onto my belly and we both started to smile. I went to bed afterwords.
My girlfriend was asking us the next day whether we had sex, and these questions are never innocent. We both turned red, but I saved the situation when I told that I masturbated in the morning under my blanket, thinking that she was still sleeping, but that I found later she was not. She happily confirmed story. She told she woke up when I grabbed the tissue to clean myself. Actually I did it really in the morning but she didn’t figure out, because she was still sleeping.