Boys probably worry about shaving their eyebrows and suddenly becoming homosexual.
Guys don't.
Let me simplify......
Either one probably picks their nose. Never admit it though!
Both like explosions.
And to finish it off....... In the words of Jeff Foxworthy......
They both just want a beer and to see something nekkid!
Seriously, when I hear "Boy" adressed towards an adult, I instantly transfer thoughts to picturing an angry redneck say (Heavy southern accent) "BOY! You get back here! Imma gunna beat you summthin fierce!"
Guy is more of a replacement for dude I guess you could say. When you don't know the person, likely you'll say "Hey guy", rather than "Hey boy".
The Flanker is gone.