I know everyone is tooling (no pun intended) on the dude who asked the question, but if you think about it from a certain perspective, it is an interesting one.
Humans evolved the largest penises of any primate. There must be a reason for this.
Was it because the males with the larger penises were providing the most pleasure to the females and thus being sexually selected by them? Was it a form of competition among the males and thus the males with the larger penises earned more 'respect' and deference when they were strutting about naked from other males for most of our evolutionary history (telling how taboo an exposed penis is in the days of pants and clothing)? Probably both. Why rule out 50% of the human population when considering the role of natural selection in how we came to be?
So we guys tend to "respect" other guys with bigger cocks, and the gals tend to just find them hot and if they also find them interesting in other respects, it just adds to their virtues in the process of sexual selection, including the built in "leader of other men" attraction trigger...
Actually, since you mention it, I have read that evolutionary biologists believe that large cocks evovled not because of sexual selection, but because men respect men with larger cocks and that they were afforded greater social status, hence they had more access to women and that the only limitation to how large penis size got was whether or not it still fit inside the vagina. You can see this behavior anthropologically with all the cultures that emphasize the size of the man's penis using gourds, with that size differential directly relating to each man's social standing.
So you started off with the right question about how men evolved such impressive penis sizes, but you assumed that women are the reason why men's penises are so much larger than the penises of other animals, instead of social selection by men.
From my casual observation of the behavior of both men and women here on LPSG and observing the fact that out of nearly 50,000 members, so very few of them are female, it appears that men are still wired to obsess about penis size, and are much more likely to "respect" another man for having a larger penis. (Of course, polls here are skewed and that must be taken into account, since visitors to LPSG are obviously more interested in penises than the general populace for whatever reason, whether it be that you are hung yourself, you are dating someone hung, you want to fuck a large cock, or you want one... but the observation of how few females there are here at LPSG can't be insignificant considering how rare extremely large penises are rendering each hung man equal to at least 4-50 women apiece depending on size, and the fact that LPSG provides women with the opportunity to be anonymous, which would encourage women to be more lurid and flagrant in their expressed opinions. If women really did desire large penises that much, this place would be crawling with horny women. The incredible absence of women here is extremely significant.)
