I have been convinced over time, that it has something to do with porn addiction. Only because every man that seems completely enthralled with his own self, and thinks every woman is too, has also turned out to be addicted to pornography.
Since I have never seen any porno of any kind, I have no idea what's in them, but I would assume that in pornos the women must pretend to be completely turned on by a guy talking about his "size" and yanking himself out for everyone to see.
Just a guess... but it fits my experience with these porno addicted. I personally have yet to meet a single girl anywhere that was impressed with "oh it's 4.5 inches!" or whatever... not even sure what response I supposed to be had when guys talk about themselves.