Actually, I think it just has to do with the fact that
sex education in the
United States is so terribly lacking. Guys have this silly idea that size is the most important thing, when really it isn't. A monster
sausage is not going to do anything for a woman if the guy it's attached to thinks he can just ram it in and go. On the other hand (or should I say
in the other hand?), a man who is highly skilled in use of his hands and tongue will have very happy lovers even if he can't even get it up.
But my point about sex ed is this: First of all, most people don't know what penises really look like except for their own and maybe a few other guys they've seen, almost always while flaccid. They probably also don't realize that a penis is NOT proportional to any other body part, and a particular penis's dimensions and appearance while erect and while flaccid have nothing to do with each other.
I took a sex ed class (under the purview of psychology) in which we actually saw a slide show of different penises, erect and flaccid, circumcised and not. I expect it was quite enlightening to much of the class. We also watched a slide show of pictures of the vulva, but that's a topic for a different node.