I can't believe nobody mentioned Republicans Vs. Democrats! Oh, wait, we were talking about things that are actually different... My bad. But back to the question. The reasons for red vs. blue are natural, conditional, and psychological. In nature, blue and red are on opposite sides of the section of the electromagnetic spectrum occupied by natural light. On a rainbow, they appear on opposite sides. Likewise, most of us are actually conditioned to view blue and red as opposites by entertainment media (fire/hot vs. ice/cold), the RBG color scheme (CMYK is better!) and myriad cultural influences, some which you mentioned. Lastly, it has to do with the psychological affect these colors have on us. Red tends to evoke passion, while blue tends to induce relaxation (psychological opposites). Orange and red are also supposed to increase appetite. I'm more of a green guy, myself. No idea what emotion that incites.