My experience with "showing" may be a bit different from the rest. My wife and I were good friends with another couple. Once, in conversation, my wife told her friend that I was uncut. The other woman replied that she had never seen an uncut penis, but wished that she could.
My wife asked me if I would let "Mary" see my penis and I said "sure" so when she and her husband were both there I came in dressed only in loose shorts, and my wife told her she could look at me if she wanted to. She said "Yes!" so I removed my shorts and stood there naked while she looked and my wife explained how I was different.
I could tell that my friend was a bit jealous that his wife had seen my cock, but that my wife had not asked to look at his. So I suggested that to be "fair" he should show his to my wife, too, which he was only too glad to do.
This was a case where the wives saw each others' husbands naked, but the reverse did not happen. My friend has admitted that his wife has mentioned the incident a few times since, and my wife has also referred to seeing "Jim" naked.
It seems to have been an important incident in their lives, and I am very much aware that it somehow made our friendship even closer.