We used to rent a small flat during the second year at the university, and for several months we lived there, two couples in just one room. We were good friends, very open so we didn´t have a problem to have sex when dark. We were not hiding any sounds but never openly watched each other, even my girl and I were seen by the other couple because we liked the idea of being caught and did it many times uncovered and completely naked. I saw them naked once, but nothing more. Later everyone of us admitted also masturbation when sleeping without a partner while the others were having sex, for girls this was easier while they didn´t make much noise, but as for me, before I started I always waited till they were finished and sleeping, so I wouldn´t get caught.
I stayed there once with the friends girlfriend. I expected her return from some evening meeting little later so I was just jerking off to some porn on my PC when she came in. I had enough time to hide myself though it was somehow obvious what I was doing. She didn´t say anything but it left me very horny. I really didn´t want to wait until she fell asleep nor a quick session in the bathroom, so just before the dark when we lay in our beds I asked her openly whether she would mind if I jerked off there and then. She was ok with it so I only noticed that she could join me and that I know she had done it before in the same room with me as well as I.
So finally we ended naked on our beds, watching each other doing ourselves. She came first and then she told me she wanted to see me cum so I ended kneeling between the beds and squirting onto the floor. It was very exciting being watched by her. I masturbated later again that night but she didn´t notice. We never got back to this story, my girlfriend wouldn´t like the fact I did it in front of somebody else. But since then I masturbated more openly when alone with the other couple during them having 'fun'.