I've observed this with other groups at times. But perhaps since I'm not in these other groups, I don't really see it as much. To me it seems more pronounced with my friends. So.....why? Why this competition?
Biologically, does it make sense for guys and girls to compete? Not really. Back in the caveman days, they had very different roles. It would make much more sense for girls and guys to cooperate. And for those of the same gender to compete with each other. Let's take a look at roles through history. Throughout history, in most societies, men and women had very different spheres of influence. Men were typically dominant in the public sphere, where there were relatively few women. Women, on the other hand were queen in their own households, in the private sphere. I am, of course, speaking of the few middle class to elite members of society. In the poorer classes, gender roles were much less distinct. In Western society, these distinctions lasted well into the twentieth century. It was not until the interwar period that women were permitted to vote. It wasn't until the 1960's that women really became an accepted part of mainstream public life. Since then it's been a slow climb for women to reach an equal footing in the public sphere with men, which of course, they still do not have yet.
In the twentieth century, you see an expose of gender inequality. Women are outraged, and men are....either righteous or embarassed of the legacy, depending on the man. But in any case, it comes down to this. Women want to prove that they can be as good as men. Men want to prove that their position as the dominant figure in man-woman relationships through the centuries was justified. There is no sense of the genders complementing each other, of yin-yang (although I use that term in the loosest way possible, because I don't really know what it means). The idea now is that women compete with men, yin-yang becomes yang-yang, or maybe yin-yin, whichever suits you. This attitude starts at an early age, particularly for those born after the 1970's. Maybe it should be this way, and maybe it shouldn't, I don't know. What I do know is that it causes a great deal of conflict between the genders.
There is a strong societal influence to identify with one's own gender. Women have to bond together. Men have to bond together. Guys night out, girls night out. And what happens during these exclusive events? Typically, a lot of talking about the opposite sex. Whether it be slamming them, admiring them, lusting after them, making battle plans against them. We all do it, and I wasn't kidding about the battle plans. Make no mistake about it, this is a battle. Guys, have you ever been in a room full of girls? Does it make you uncomfortable? Not even full of girls. If you're outnumbered more than 4 or 5 to 1, even 3 to 1 sometimes, you start to get either bored or uncomfortable. And if you don't.....well, maybe you're on their side eh? I'm sure that it's the same for girls. This isn't an absolute rule, there are lots of exceptions. But it's definitely a pattern. If there were equality, then it shouldn't matter who are guys and girls. Gender shouldn't come in into play at all.
Gender, I think, is one of the most divisive issues ever. Particularly in the Western world, where Puritan America has shaped the world in it's image -- that of a ultra conservative, and (this is key), a anti-sex image. Sex taboo means gender mysteries and puzzles. But that's another topic for another day.