Wait, adding vouyerism to something not allowed, and making it merely observed makes it OK?
I think the word "intent" comes into play. I know the difference between voyeurism and a look. The OP asks "how many have seen" not how many have spied or tried to look. Of course, throwing in an adjective of "horsehung" sort of colors the request, but I'm going with verbs here.
I've seen a male relative's penis. It did not excite me; I did not stare; it was accidental, and I excused myself and left. That is not incest, is not does excite prurient interest in me, and the closest thing sexual about it was that it was his sexual organ.