Think everyone's a little bisexual. With girls it's supported for them to find other women attractive, hence I'm not really surprised a lot put themselves as "bisexual". Whereas for guys in the media generally it's unpopular which is why guys will hardly say they are a bit bisexual. The media is incredibly powerful really. I'm sure if it was cool for men to be bisexual, you'd see a lot more men bisexual.
Fact is most people can recognise a good looking person regardless of sex. And good looking people can be nice to look at. Even for guys this is true, it's just we don't tend to admit it. While for females it's even cool to admit it.
Fact is most people however have one gender they are attracted to far more than the other. A lot of girls might find the odd girl attractive but find more men sexually attractive. Especially a lot of girls who despite finding women attractive, would not bring themselves to actually have a long term relationship or have sexual relations with a girl. So really for all purposes, they're really straight like most people really, as explained earlier, I don't think anyone mostly is 100% straight.
Bisexual is a confusing word, as you could describe almost anyone as it, if anyone finds the occasional person of the same sex attractive as well as finding the opposite sex attractive. Not uncommon. I think it's rarer those who are sexually attracted to men and women in more even ratios e.g. 50:50. If we really wanted to distinguish between them, maybe we need a different word. But then again, there probably isn't much need to distinguish between them anyway, they're all people at the end of the day.