This happened 20 years ago. My brother, sister in law, wife and I shared wives a few times. I visited my sister in law and brother one afternoon but he was not at home and she was getting ready to bathe. I wanted to talk a bit so she said come into the bathroom as the tub is ready. She dropped her robe as she walked down the hall and I followed that lovely behind until it disappeared in to the tub. Watching her wash herself, especially her breasts, caused me to get hard quite evidently. She began to smile and said you need to do something about that before you walk out to your car. Nosy neighbor lady will gab all over the neighborhood.
Well as I was sitting on the toilet anyway I said I was hoping you'd say that. I pulled down my pants and out came my rock hard cock. As I started to stroke myself she stopped bathing, sat back with a smile on her face and urged me on. It didn't take long as she was talking really hot to me. As she oohed and aahed and said stroke it, come on stroke it I told her I was cumming. I turned towards the tub and my cum hit the wall. This was one of many times she watched and sometimes helped me get off.