I think what happens is two things. First off, porn is the main offender, but this isn't news, considering the average penis in porn is significantly larger than the real-life average. This is what I really think is the catalyzer, though: I, as a straight guy, have been conditioned my whole life to look away as quick as possible if I were exposed a penis. This is something every guy can relate to (if they're young and from my generation, at least): If a friend ran through the lockerroom naked, you look the other way and yell "ugh!". If you're on a computer with your friends and a penis is flashed for whatever reason, you all look the other way and yell "ugh!". Basically, you're taught from day one not to ever look at a penis. Because of this, the only time guys will (admittedly) see a penis is when he's seeing one plow a girl in his favorite porn. These penises are the only ones that register in his mind, so he uses that as a reference, even if his penis is much smaller than what he would consider "normal" due to this. I remember being that way. I saw the leaked Fred Durst video and thought he had a below-average penis. I remember guessing "he must be 4 or 5 inches". I recently saw it again (after being educated on what REAL average size is) and I realize he is actually pretty average, maybe a bit on the smaller end at worst. I also remember not really knowing where I stood on the whole size thing. I remember sending pictures of my dick to girls online and them thinking it was big. I remember some saying if was small. I was torn between the two, although I never realized it because I never gave it prolonged thought. Sometimes I was genuinely under the impression that I was big and other times the total opposite. The penis and its attributes are something totally barred out from society due to the stigmas related to it. It's too awkward to discuss and compare. No one feels like talking about it so instead they invented a rule that if you didn't look away and "Ugh!" at a penis (unless you were privately jerking off to porn, of course), it means you're a "fag".