Is it normal for a man to like to see his wife strip for other men?

Soon after our marriage my wife and I were skinny-dipping at what we thought was a private spot on a river. I got out before she did and had put my clothes on and while she was still floating on her float that was anchored to a tree at the edge of the river. She was lying on her back getting an all over tan. While I had walked back to our car to get her a towel several guys who had apparently been walking along the river bank were walking straight toward where she was floating. Instead of warning her I stepped back and watched them walk to the spot where she was still floating. They stopped and stared at her naked body and I found myself getting sexually aroused. She must have heard them and got off the float and went under the water. When they saw me returning they walked on and and she got out of the water and dressed. I told her I enjoyed the guys seeing her and she said that she really didn't mind and kind of got excited thinking about it. That was our first such experience but not our last. After 30 years of marriage that was not the last time other men saw her naked and we progressed in the intensity of what we did together with other men, or what she did with other men while I was there. Is it normal for for a couple who love each other very much to have enjoyed what others might think is abnormal?