This happened to me several years ago.
I stopped in a peep show one day after work to see a few sexy movie clips. I went into the booth and put some quarters in the slot. As I was enjoying the movie a guy came to the booth door and asked in a whispered voice if he could 'help me.' That puzzled me a bit as I wasn't sure what he meant, but I was game so I said, 'sure, come in.' He did.
We were watching the movie for a few minutes when suddenly I felt a hand gently and unexpectedly touch my groin area. I wasn't too much surprised since we were in a booth watching a beautiful chick rub herself in the movie. So I just sorta let it happen. After a few minutes of this he asked if he could unzip my fly. Since his touching felt so exciting I said, 'sure, go ahead.' He did, then gently maneuvered his hand into the fly and began to fondle my now semi hard cock. I could feel that he was groping for the slit in my shorts to get to my cock. I let him take my cock out of my shorts and pants fly. Then he went back in for my balls which I let him pull out. By now I was completely hard and was enjoying the exposure and fondling.
With my cock and balls fully exposed he began to have his way with me sliding his hand slowly up and down my shaft, bringing the foreskin to over the head to spread lubrication now dripping from the end. Then I heard his pants unzip. I reached down to his groin area and found his cock and balls out of his pants. The end of his cock was dripping wet. We stood there watching the lady on the screen masturbate herself while we played with each other. In a few minutes the film ended and the light in the booth came on. We both zipped up and left. We never did masturbate each other to ejaculation. Just played with each other.
I hope this happens to me again at which time I think I will suggest more vigorous masturbation.