First off the social convention is not to stare, and even sneaking a peek is not really acceptable (though guys do of course). However there's an even more powerful social convention which is to pretend you haven't seen. For example a guy in my gym has a butterfly tattooed on his bum. There's no missing it. There is no way I could ever ask him why as this would be me saying I've seen it when the social convention is to pretend that I have not.
I may have mentioned this in an earlier thread -- While at an event in town, I ran into a retired guy I often chat with in my gym's locker room. He smiled and said "Funny seeing you with clothes on!" Great sense of humor.
I've been a regular at the same gym for about 8 years now and though it's a relaxed, friendly place, I have never once heard anyone mention a single thing that was sexual in nature nor would I expect to.