My girlfriend and I are usually the ones who are leaving our local gym last, at 10pm. Just after exercising we take a shower together - it´s divided into a men and women part, but since we are the only ones in the place we shower naked together. They have never told us it was wrong, so we do it all the time.
About two weeks ago another couple of about our age stayed there almost to the end, they left just minutes before us. My girlfriend went to the restrooms first, and I went to the men´s shower. To my surprise the other couple was showering there together as we planned to do. Even though it´s just one open room with no divisions I didn´t mind, as we did it all the time there too. I only thought that my girlfriend wouldn´t join us when she would find we were not alone.
In about a minute I turned a bit to see if they were still there and found out they were touching each other in an intimate way. They obviously didn´t mind my presence there, so I could see quite a lot and it brought me to a pretty intense hardon. When my girlfriend appeared first she wanted to leave to another shower, but then she saw the situation as well as my excitement and came to me.
She grabbed my dick and started to do me a handjob while I was touching her on her clit. Soon we watched the other couple and it was amazing - they were doing almost the same, aroused by us too. I had a very strong ejaculation in front of them and I wanted my girl to have an orgasm too, but she stopped me telling me she rather wanted me inside her - at home. (We had great sex afterwards.)
We haven´t met the couple again yet but I still think about this when I masturbate.