Richard Gere in American Gigolo. Apparently, the famous nude scene was unscripted, he decided that it would make more sense for the character to be nude, so he was.
After I posted, I read that John Travolta and Christopher Reeves were both offered the part. I wonder if they would have done the same thing,
Since most men are actually quite average down there, I would daresay most actors are too. And probably don't want to put it out there for that reason.
I remember Bernadette Peters once said she didn't want to do a nude scene because then the convo would turn toward her breasts rather than the movie.
Likewise, an actor takes his clothes off and it becomes about his penis.
It seems most of the guys who have done it have been more endowed, which is why you know they do it.
If they were average or smaller they wouldn't want that convo going around.
And being that the penis is the sex organ on a male, and is plain to see, I can see why men would be sensitive to it.