Good morning.
"Do guys really not know that stuff is hanging out?"
I hate to admit it, but we really are that unaware. Unless there's a cold breeze, or someone is kind enough to let us know. With more men wearing running shorts and the like, as the warmer weather hits, you're bound to see it more often.
Of course, I won't deny there are exhibitionists out there who are doing it on purpose for the thrill or shock value.
The best thing you can do is either ignore it, or tell him. Just don't be nasty or mean about it. A simple "You might want to put that away," or "You know, you're hanging out," will suffice. Most men will react immediately by stuffing things back in where they belong and offering a weak or embarrassed apology.