"That guy" refers to the narcissist inside each and every man. "He" is "That guy" when he gives into the urges (usually pertaining to sexual gratification) to the detriment of those around him. Nobody wants to be "That guy".
Bob: "Dude... don't freak... I got a blowjob from your sister last night ... but I don't want to be 'That guy'".
Another Bob: "Look at 'That guy' next to 'prince cassius' in the fashion section of the evening standard".