the phrase itself derived from the Girl Next Door...
-There is this really hot girl, the girl next door
=So did you bang her yet?
-Yeah, right I banged her (sarcasm)
because "THATS WHAT A MAN DOES."
Lines that are associated with "what a man does":
"All I wanna do is bang hot chicks" (girl next door)
"We are all going streaking...through the quad to the gymnasium" (old school)
"F#CK IT, I'll squeeze one" (GND)
"We can't lose our composure" (OS)
"You got to use my girlfriend, shes so good, wanna give her a throw?" (GND)
"I guess I am a little freaked out about only having sex with one person (points to his wife) for the rest of my life" (OS)
"How's the rack?" (GND)
"Do you know how lucky you are to land someone as sexually enlightened as Hedi..look Columbus wasn't looking for America when he went sailing away, but he found it and that sure as heck worked out for the rest of us" (0S)
"I would have nailed her right there and then"(GND)
FOR INFO ON HOW TO DO WHAT A MAN DOES: Learn from anything that a man has done in the Girl Next Door (especially giving her Bourbon to get her drunk for sex) and Old School. These are textbook examples of "what a man does."